As filed with the Securities and Exchange Commission on August 15, 2018

Registration No. 333-226211

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

Amendment No. 1

to

FORM S-4

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

BOK FINANCIAL CORPORATION

(Exact name of registrant as specified in its charter)

Bank of Oklahoma Tower

Boston Avenue at Second Street

Tulsa, Oklahoma 74192

(918) 588-6000

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Frederic Dorwart

Corporate Secretary and General Counsel

124 East 4^th Street

Tulsa, Oklahoma 74103

(918) 583-9922

(Name, address, including zip code, and telephone number, including area code, of agent for service)

With copies to:

Approximate date of commencement of proposed sale of the securities to the public: As soon as practicable after this Registration Statement becomes effective and upon completion of the merger described in the enclosed proxy statement/prospectus.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box.  ☐

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act of 1933, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

Exchange Act Rule 13e-4(i) (Cross-Border Issuer Tender Offer)  ☐

Exchange Act Rule 14d-1(d) (Cross-Border Third-Party Tender Offer)  ☐

CALCULATION OF REGISTRATION FEE

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

Information contained herein is subject to completion or amendment. A registration statement relating to these securities has been filed with the Securities and Exchange Commission. These securities may not be sold nor may offers to buy be accepted prior to the time the registration statement becomes effective. This proxy statement/prospectus shall not constitute an offer to sell or the solicitation of an offer to buy nor shall there be any sale of these securities in any jurisdiction in which such offer, solicitation or sale is not permitted or would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction.

PRELIMINARY PROXY STATEMENT/PROSPECTUS

DATED JULY 17, 2018, SUBJECT TO COMPLETION

MERGER PROPOSED—YOUR VOTE IS VERY IMPORTANT

[●], 2018

Dear Shareholder:

On June 17, 2018, CoBiz Financial Inc. (which we refer to as “CoBiz”) entered into an Agreement and Plan of Merger (which we refer to as the “merger agreement”) with BOK Financial Corporation (which we refer to as “BOK”). Under the merger agreement, CoBiz will merge with and into BOK Merger Corporation Number Sixteen (which we refer to as “Merger Sub”), with Merger Sub as the surviving corporation, in a transaction that we refer to as the “merger”. Immediately following the merger, or at such later time as BOK may determine, CoBiz’s wholly-owned subsidiary, CoBiz Bank, will merge with and into BOK’s wholly-owned subsidiary, BOKF, National Association, with BOKF, National Association as the surviving bank.

Under the terms and conditions of the merger agreement, each shareholder of CoBiz, as of the record date, will have the right to receive 0.17 shares of BOK common stock and $5.70 in cash for each share of CoBiz common stock (which we refer to as the “merger consideration”) held immediately prior to the merger. The value of the merger consideration will depend on the market price of BOK common stock on the effective date of the merger.

Based on the closing price of BOK common stock on the NASDAQ Global Select Market (which we refer to as “NASDAQ”) on June 15, 2018, the last trading day before public announcement of the merger, the value of the per share merger consideration payable to holders of CoBiz common stock would be $23.02. Based on the closing price of BOK common stock on NASDAQ on August 14, 2018, the last practicable trading date before the date of the attached proxy statement/prospectus, the value of the per share merger consideration payable to holders of CoBiz common stock would be $22.46. Based on the number of shares of CoBiz common stock outstanding and the number of shares of CoBiz common stock issuable pursuant to outstanding CoBiz stock options and restricted stock awards, in each case as of August 14, 2018, the total number of shares of BOK common stock expected to be issued in connection with the merger is approximately 7.2 million. In addition, based on the number of issued and outstanding shares of BOK common stock and CoBiz common stock on August 14, 2018, and based on the exchange ratio of 0.17, holders of shares of CoBiz common stock as of immediately prior to the closing of the merger will hold, in the aggregate, approximately 9.9% of the issued and outstanding shares of BOK common stock immediately following the closing of the merger (without giving effect to any BOK common stock held by CoBiz’s shareholders prior to the merger).

CoBiz will hold a special meeting of its shareholders in connection with the merger. CoBiz’s shareholders will be asked to vote to approve the merger agreement and approve related matters, as described in the attached proxy statement/prospectus. The merger cannot be completed unless, among other things, the holders of at least a majority of the outstanding shares of CoBiz common stock entitled to be cast on the proposal approve the merger agreement.

The special meeting of shareholders of CoBiz will be held at CoBiz’s office at 1401 Lawrence Street, Suite 1200, Denver, Colorado 80202 on September 27, 2018 at 8:00 a.m., local time.

CoBiz’s board of directors has unanimously approved and adopted the merger agreement and the transactions contemplated thereby and recommends that you vote “FOR” the approval of the merger agreement and “FOR” the other matters to be considered at the special meeting.

The attached proxy statement/prospectus, which serves as the proxy statement for the special meeting of the shareholders of CoBiz and the prospectus for the shares of BOK common stock to be issued in the merger, includes detailed information about the special meeting, the merger and the documents related to the merger. We urge you to read the entire proxy statement/prospectus carefully, including the discussion of the risks related to the merger and owning BOK common stock after the merger in the section titled “Risk Factors” beginning on page [22]. You can also obtain information about CoBiz and BOK from documents that have been filed with the Securities and Exchange Commission that are incorporated in the proxy statement/prospectus by reference.

Shares of BOK common stock are listed on NASDAQ under the symbol “BOKF.” Shares of CoBiz common stock are listed on NASDAQ under the symbol “COBZ.” On August 14, 2018, the last practicable trading day prior to the date of the attached proxy statement/prospectus, the last sales price of BOKF common stock was $98.59 per share, and the last sales price of CoBiz common stock was $22.16 per share. You should obtain current market quotations for both BOK common stock and CoBiz common stock.

Your vote is important. Whether or not you plan to attend the special meeting, please take the time to vote by completing and mailing the enclosed proxy card or by submitting a proxy through the Internet or by telephone as described on the enclosed instructions as soon as possible to make sure your shares are represented at the special meeting. If you submit a properly signed proxy card without indicating how you want to vote, your proxy will be counted as a vote “FOR” the approval of the merger agreement and “FOR” the other matters to be considered at the special meeting. The failure to vote by submitting your proxy or attending the special meeting and voting in person will have the same effect as a vote against approval of the merger agreement. Submitting a proxy now will NOT prevent you from being able to vote in person at the special meeting. If you hold your shares in “street name,” through a bank, broker or other nominee, you should instruct your bank, broker or other nominee how to vote in accordance with the voting instruction form you receive from your bank, broker or other nominee.

Sincerely,

Steven Bangert

Chairman of the Board and Chief Executive Officer

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the securities to be issued in the merger or passed upon the adequacy or accuracy of this proxy statement/prospectus. Any representation to the contrary is a criminal offense.

The shares of BOK’s common stock to be issued in the merger are not savings accounts, deposits or other obligations of any bank or savings association and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency.

This proxy statement/prospectus is dated [●], 2018, and is first being mailed to CoBiz’s shareholders on or about [●], 2018.

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS

[●], 2018

To the Shareholders of CoBiz Financial Inc.:

Notice is hereby given that a special meeting of shareholders of CoBiz Financial Inc., a Colorado corporation (which we refer to as “CoBiz”), will be held at CoBiz’s office at 1401 Lawrence Street, Suite 1200, Denver, Colorado 80202 on September 27, 2018 at 8:00 a.m., local time, for the purpose of considering and voting upon the following matters:

CoBiz’s board of directors has fixed the close of business on August 14, 2018 as the record date for determination of shareholders entitled to notice of and to vote at the special meeting, and only shareholders of record on said date will be entitled to receive notice of and to vote at said meeting.

The affirmative vote of the holders of at least a majority of the outstanding shares of CoBiz common stock entitled to be cast on the proposal at the special meeting is required to approve the merger proposal. Approval of the compensation proposal and approval of the adjournment proposal require the affirmative vote of the holders of at least a majority of the votes cast at the special meeting. CoBiz’s shareholders must approve the merger proposal in order for the merger to occur. CoBiz’s shareholders are not, however, required to approve the compensation proposal or the adjournment proposal in order for the merger to occur. If CoBiz’s shareholders fail to approve the compensation proposal or the adjournment proposal, but approve the merger proposal, the merger may nonetheless occur.

CoBiz’s board of directors has unanimously approved and adopted the merger agreement and the transactions contemplated thereby and recommends that you vote “FOR” the merger proposal, “FOR” the compensation proposal and “FOR” the adjournment proposal.

By Order of the Board of Directors

Richard J. Dalton

Corporate Secretary

Your vote is important. Whether or not you plan to attend the special meeting, please take the time to vote by completing and mailing the enclosed proxy card or by submitting a proxy through the Internet or by telephone as described on the enclosed instructions as soon as possible to make sure your shares are represented at the special meeting. If you submit a properly signed proxy card without indicating how you want to vote, your proxy will be counted as a vote “FOR” the merger proposal, “FOR” the compensation proposal and “FOR” the adjournment proposal. The failure to vote by submitting your proxy or attending the special meeting and voting in person will have the same effect as a vote against approval of the merger agreement. Submitting a proxy now will NOT prevent you from being able to vote in person at the special meeting. If you hold your shares in “street name,” through a bank, broker or other nominee, you should instruct your bank, broker or other nominee how to vote in accordance with the voting instruction form you receive from your bank, broker or other nominee.

ADDITIONAL INFORMATION

The accompanying proxy statement/prospectus incorporates by reference important business and financial information about BOK and CoBiz from documents filed with the Securities and Exchange Commission (which we refer to as the “SEC”) that are not included in or delivered with the proxy statement/prospectus. You can obtain any of the documents filed with or furnished to the SEC by BOK or CoBiz at no cost from the SEC’s website at http://www.sec.gov. The proxy statement/prospectus is available for viewing, printing and downloading at www.envisionreports.com/COBZ. Additionally, you may also request copies of these documents, including documents incorporated by reference in the proxy statement/prospectus, at no cost upon written or oral request by contacting the appropriate company at the following addresses and telephone numbers:

You will not be charged for any of these documents that you request. To obtain timely delivery of these documents, you must request them no later than five business days before the date of the CoBiz special meeting. This means that CoBiz’s shareholders requesting documents must do so by September 20, 2018, in order to receive them before the special meeting.

For a more detailed description of the information incorporated by reference in the accompanying proxy statement/prospectus and how you may obtain it, see “Where You Can Find More Information,” beginning on page 102 of the accompanying proxy statement/prospectus.

The accompanying proxy statement/prospectus provides a detailed description of the merger and the merger agreement. We urge you to read the accompanying proxy statement/prospectus, including any documents incorporated by reference into the accompanying proxy statement/prospectus, and its annexes carefully and in their entirety.

If you have any questions concerning the merger, the other matters to be considered at the special meeting or the accompanying proxy statement/prospectus or need assistance voting your shares, please contact CoBiz’s proxy solicitor at the address or telephone number listed below:

MacKenzie Partners, Inc.

1407 Broadway, 27^th Floor

New York, NY 10018

Please do not send your stock certificates at this time. You will be sent separate instructions regarding the surrender of your stock certificates.

You should rely only on the information contained in, or incorporated by reference into, the proxy statement/prospectus. No one has been authorized to provide you with information that is different from that contained in, or incorporated by reference into, the proxy statement/prospectus. The proxy statement/prospectus is dated [●], 2018, and you should assume that the information in the proxy statement/prospectus is accurate only as of such date unless information specifically indicates that another date applies.

The proxy statement/prospectus does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction in which or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction.

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RISK FACTORS

In addition to the general investment risks and the other information contained in or incorporated by reference into this proxy statement/prospectus, including the matters addressed under the caption “Information Regarding Forward-Looking Statements,” beginning on page 29, you should carefully consider the following risk factors in deciding whether to vote for approval of the merger agreement. You should also consider the other information in this proxy statement/prospectus and the other documents incorporated by reference in this proxy statement/prospectus. For more information, see “Where You Can Find More Information,” beginning on page 102.

Risks Related to the Merger and BOK’s Business upon Completion of the Merger

Because the market price of BOK common stock will fluctuate, CoBiz’s shareholders cannot be certain of the market value of the merger consideration they will receive.

Upon completion of the merger, each outstanding share of CoBiz common stock (except for specified shares of CoBiz common stock held by CoBiz or BOK (which will be cancelled)) will be converted into the right to receive 0.17 shares of BOK common stock and $5.70 cash. The market value of the BOK common stock to be issued in the merger will depend upon the market price of BOK common stock. This market price may vary from the closing price of BOK common stock on the date the merger was announced, on the date that this proxy statement/prospectus was mailed to CoBiz’s shareholders and on the date of the CoBiz special meeting. There will be no adjustment to the merger consideration for changes in the market price of either shares of BOK common stock or CoBiz common stock.

The market price of BOK common stock could be subject to significant fluctuations due to changes in sentiment in the market regarding BOK’s operations or business prospects, including market sentiment regarding BOK’s entry into the merger agreement. These risks may be affected by:

Accordingly, at the time of the CoBiz special meeting, CoBiz’s shareholders will not necessarily know or be able to calculate the value of the merger consideration they would be entitled to receive upon completion of the merger. You should obtain current market quotations for both BOK common stock and CoBiz common stock.

The market price of BOK common stock after the merger may be affected by factors different from those affecting the shares of BOK or CoBiz currently.

The businesses of BOK and CoBiz differ, and accordingly, the results of operations of the combined company and the market price of the shares of BOK’s common stock after the completion of the merger may be affected by factors different from those currently affecting the independent results of operations and market prices of common stock of each of BOK and CoBiz. For a discussion of the businesses of BOK and CoBiz and of certain

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factors to consider in connection with those businesses, see the documents incorporated by reference in this proxy statement/prospectus and referred to under “Where You Can Find More Information,” beginning on page 102.

The success of the merger and integration of BOK and CoBiz will depend on a number of uncertain factors.

The success of the merger will depend on a number of factors, including:

Integrating the acquired business will be an operation of substantial size and expense and may be affected by general market and economic conditions or government actions affecting the financial industry generally. Integration efforts will also likely divert BOK’s management’s attention and resources. No assurance can be given that BOK will be able to integrate the acquired business successfully. Additionally, no assurance can be given that the operation of the acquired business will not adversely affect BOK’s existing profitability, that BOK will be able to achieve results in the future similar to those achieved by its existing banking business or that BOK will be able to manage any growth resulting from the merger effectively.

Regulatory Approvals May Not Be Received, May Take Longer than Expected or May Impose Conditions that Are Not Presently Anticipated or Cannot Be Met.

Before the transactions contemplated in the merger agreement can be completed, various approvals must be obtained from the bank regulatory and other governmental authorities. In deciding whether to grant these approvals, the relevant governmental entities will consider a variety of factors, including the regulatory standing of each of the parties, the effect of the merger on competition, and the factors described in the section of this proxy statement/prospectus entitled “Regulatory Approvals Required for the Merger.” An adverse development in either party’s regulatory standing or other factors could result in an inability to obtain one or more of the required regulatory approvals or delay receipt of required approvals. The Federal Reserve Board has stated that if supervisory issues arise during processing of an application for approval of a merger transaction, a banking organization will be expected to withdraw its application pending resolution of such supervisory concerns. No assurance can be given that the OCC would not also require a similar withdrawal of an application if supervisory issues arise during processing of an application for approval of a bank merger transaction. Accordingly, if there is an adverse development in either party’s regulatory standing, BOK or BOKF Bank may be required to withdraw its application for approval of the proposed merger and bank merger, as applicable, and, if possible, resubmit such application after the applicable supervisory concerns have been resolved.

The terms of the approvals that are granted may impose conditions, limitations, obligations or costs, or place restrictions on the conduct of the combined company’s business or require changes to the terms of the transactions contemplated by the merger agreement. There can be no assurance that regulators will not impose any such conditions, limitations, obligations or restrictions and that such conditions, limitations, obligations or restrictions will not have the effect of delaying the completion of any of the transactions contemplated by the merger agreement, imposing additional material costs on or materially limiting the revenues of the combined

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company following the merger or otherwise reduce the anticipated benefits of the merger if the merger were consummated successfully within the expected timeframe. Nor can there be any assurance that any such conditions, terms, obligations or restrictions will not result in the delay or abandonment of the merger. Additionally, the completion of the merger is conditioned on the absence of certain orders, injunctions or decrees by any court or regulatory agency of competent jurisdiction that would prohibit or make illegal the completion of any of the transactions contemplated by the merger agreement.

BOK and CoBiz believe that the proposed transactions should not raise significant regulatory concerns and that BOK will be able to obtain all requisite regulatory approvals in a timely manner. However, the processing time for obtaining regulatory approvals for mergers of banking institutions, particularly for larger institutions, has increased since the financial crisis. In addition, despite the parties’ commitments to use their reasonable best efforts to comply with conditions imposed by regulatory entities, under the terms of the merger agreement, BOK will not be required and CoBiz will not be permitted to take actions that would reasonably be expected to have a material adverse effect on BOK and its subsidiaries, taken as a whole, after giving effect to the merger (measured on a scale relative to CoBiz and its subsidiaries, taken as a whole). See “Regulatory Approvals Required for the Merger” beginning on page 65.

Combining BOK and CoBiz may be more difficult, costly or time consuming than expected and the anticipated benefits and cost savings of the merger may not be realized.

BOK and CoBiz have operated and, until the completion of the merger, will continue to operate, independently. The success of the merger, which will represent BOK’s largest transaction to date, including anticipated benefits and cost savings, will depend, in part, on BOK’s ability to successfully combine and integrate the businesses of BOK and CoBiz in a manner that permits growth opportunities and does not materially disrupt existing customer relations nor result in decreased revenues due to loss of customers. It is possible that the integration process could result in the loss of key employees, the disruption of either company’s ongoing businesses or inconsistencies in standards, controls, procedures and policies that adversely affect the combined company’s ability to maintain relationships with clients, customers, depositors and employees or to achieve the anticipated benefits and cost savings of the merger. The loss of key employees could adversely affect BOK’s ability to successfully conduct its business, which could have an adverse effect on BOK’s financial results and the value of its common stock. BOK may also encounter unexpected difficulties or costs during the integration that could adversely affect its earnings and financial condition, perhaps materially. If BOK experiences difficulties with the integration process and attendant systems conversion, the anticipated benefits of the merger may not be realized fully or at all or may take longer to realize than expected. As with any merger of financial institutions, there also may be business disruptions that cause BOK and/or CoBiz to lose customers or cause customers to remove their accounts from BOK and/or CoBiz and move their business to competing financial institutions. Integration efforts between the two companies may also divert management attention and resources. These integration matters could have an adverse effect on each of BOK and CoBiz during this transition period and for an undetermined period after completion of the merger on the combined company. In addition, the actual cost savings of the merger could be less than anticipated.

The combined company may be unable to retain BOK and/or CoBiz personnel successfully after the merger is completed.

The success of the merger will depend in part on the combined company’s ability to retain the talents and dedication of key employees currently employed by BOK and CoBiz. It is possible that these employees may decide not to remain with BOK or CoBiz, as applicable, while the merger is pending or with the combined company after the merger is consummated. If key employees terminate their employment or if an insufficient number of employees is retained to maintain effective operations, the combined company’s business activities may be adversely affected, and management’s attention may be diverted from successfully integrating CoBiz to hiring suitable replacements, all of which may cause the combined company’s business to suffer. In addition, BOK and CoBiz may not be able to locate suitable replacements for any key employees who leave either company or to offer employment to potential replacements on reasonable terms.

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The merger agreement limits CoBiz’s ability to pursue alternatives to the merger.

The merger agreement contains provisions that make it more difficult for CoBiz to sell its business to a party other than BOK. These provisions include a general prohibition on CoBiz’s solicitation of any acquisition proposal or offer for a competing transaction, the requirement that CoBiz pay a termination fee of $34.2 million if the merger agreement is terminated in certain circumstances and the requirement that CoBiz submit the approval of the merger agreement to a vote of CoBiz’s shareholders even if CoBiz’s board of directors changes its recommendation in favor of the approval of the merger agreement in a manner adverse to BOK. For more information, see “The Merger Agreement—Agreement Not to Solicit Other Offers,” “The Merger Agreement—Shareholder Approval” and “The Merger Agreement—Termination Fee,” beginning on pages 77, 75 and 82, respectively.

These provisions might discourage a third party that might have an interest in acquiring all or a significant part of CoBiz from considering or proposing that acquisition, even if that party were prepared to pay consideration with a higher per share value than the proposed merger consideration. Furthermore, a potential competing acquiror may propose to pay a lower per share price to CoBiz’s shareholders than it might otherwise have proposed to pay because of CoBiz’s obligation, in connection with termination of the merger agreement under certain circumstances, to pay BOK a $34.2 million termination fee.

The merger agreement may be terminated in accordance with its terms, and the merger may not be completed.

The merger agreement is subject to a number of conditions, which must be fulfilled in order to complete the merger. Those conditions include: (1) the approval of the merger proposal by the requisite vote of CoBiz’s shareholders; (2) the receipt of all required regulatory approvals and expiration or termination of all statutory waiting periods in respect thereof; (3) authorization for listing on NASDAQ of the shares of BOK common stock to be issued in the merger; (4) effectiveness of the registration statement of which this proxy statement/prospectus is a part with respect to the BOK common stock to be issued upon the closing of the merger; (5) the absence of any order, injunction or other legal restraint preventing the completion of the merger or making the completion of the merger illegal; (6) subject to certain exceptions, the accuracy of the representations and warranties of each of BOK and CoBiz; (7) performance in all material respects by each of BOK and CoBiz of its respective obligations under the merger agreement; and (8) receipt by each of BOK and CoBiz of an opinion from its respective counsel to the effect that the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Code.

In addition, BOK may choose to terminate the merger agreement if, prior to obtaining the approval by CoBiz’s shareholders of the merger agreement, CoBiz’s board of directors (i) fails to recommend in this proxy statement/prospectus that CoBiz’s shareholders approval the merger agreement or makes a material adverse recommendation change with respect thereto or (ii) breaches in any material respect certain of its obligations to call and hold the special meeting of CoBiz’s shareholders for the purpose of voting upon the approval of the merger agreement and its obligation not to solicit other offers. For a more complete discussion of the circumstances under which the merger agreement could be terminated, see “The Merger Agreement—Termination of the Merger Agreement,” beginning on page 82.

Potential Litigation Against CoBiz and BOK Could Result in an Injunction Preventing the Completion of the Merger or a Judgment Resulting in the Payment of Damages.

Shareholders of CoBiz may file lawsuits against BOK, CoBiz and/or the directors and officers of either company in connection with the merger. These lawsuits could prevent or delay the completion of the merger and result in significant costs to CoBiz and/or BOK, including any costs associated with the indemnification of directors and officers. The defense or settlement of any lawsuit or claim that remains unresolved at the time the merger is completed may adversely affect BOK’s business, financial condition, results of operations and cash flows.

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CoBiz’s shareholders will have a reduced ownership and voting interest after the merger and will exercise less influence over management of the combined organization.

CoBiz’s shareholders currently have the right to vote in the election of CoBiz’s board of directors and on other matters affecting CoBiz. Upon the completion of the merger, each of CoBiz’s shareholders that receives shares of BOK common stock will become a stockholder of BOK with a percentage ownership of the combined organization that is much smaller than the shareholder’s percentage ownership of CoBiz. George Kaiser will continue to own more than 50% of BOK upon completion of the merger and will maintain significant control over BOK. Based on the number of shares outstanding on August 14, 2018 and the shares expected to be issued in the merger, it is expected that CoBiz’s former shareholders as a group will receive shares in the merger that equal approximately 9.9% of the outstanding shares of BOK common stock immediately after the merger. Because of this, CoBiz’s shareholders as a group will have significantly less influence on the management and policies of BOK than they now have on the management and policies of CoBiz.

Certain of CoBiz’s directors and executive officers have interests in the merger that may be different from, or in addition to, the interests of CoBiz’s shareholders generally.

In considering the information contained in this proxy statement/prospectus, you should be aware that some of CoBiz’s executive officers and directors have interests in the merger that may be different from, or in addition to, the interests of CoBiz’s shareholders generally. These interests include the vesting in full of certain outstanding CoBiz equity compensation awards, rights to continued indemnification and insurance coverage by BOK after the merger for acts or omissions occurring before the merger and serving on the boards of BOK and BOKF, National Association. These interests and arrangements may create potential conflicts of interest. CoBiz’s board of directors was aware of these interests and considered these interests, among other matters, when making its decision to approve and approve the merger agreement and in recommending that CoBiz’s shareholders vote in favor of approving the merger agreement.

For a more complete discussion of these interests, see “The Merger—Interests of CoBiz’s Directors and Executive Officers in the Merger,” beginning on page 54.

The fairness opinion delivered by CoBiz’s financial advisor to CoBiz’s board of directors prior to the execution of the merger agreement will not reflect changes in circumstances subsequent to the date of the fairness opinion.

BofA Merrill Lynch, CoBiz’s financial advisor in connection with the merger, delivered its fairness opinion to CoBiz’s board of directors on June 17, 2018. The opinion of BofA Merrill Lynch speaks only as of such date and does not reflect changes that may occur or may have occurred after the date of the opinion, including changes to the operations and prospects of BOK or CoBiz, changes in general market and economic conditions or regulatory or other factors. Any such changes may materially alter or affect the relative values of BOK and CoBiz.

The merger is subject to the receipt of consents and approvals from governmental entities that may delay the date of completion of the merger or impose conditions that could have an adverse effect on BOK.

Before the merger and the bank merger may be completed, various approvals or consents must be obtained from governmental authorities. Satisfying the requirements of these governmental entities may delay the date of completion of the merger. In addition, these governmental entities may include conditions on the completion of the merger or require changes to the terms of the merger. While BOK and CoBiz do not currently expect that any such conditions or changes would result in a material adverse effect on BOK, there can be no assurance that they will not, and such conditions or changes could have the effect of delaying completion of the merger or imposing additional costs on or limiting the revenues of BOK following the merger, any of which might have a material adverse effect on BOK following the merger.

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CoBiz and BOK will be subject to business uncertainties and contractual restrictions while the merger is pending.

Uncertainty about the effect of the merger on employees and customers may have an adverse effect on CoBiz or BOK. These uncertainties may impair CoBiz’s or BOK’s ability to attract, retain and motivate key personnel until the merger is completed and could cause customers and others that deal with CoBiz or BOK to seek to change existing business relationships with CoBiz or BOK. Retention of certain employees by CoBiz or BOK may be challenging while the merger is pending, as certain employees may experience uncertainty about their future roles with the combined company. If key employees depart because of issues relating to the uncertainty and difficulty of integration or a desire not to remain with CoBiz or BOK, CoBiz’s business or BOK’s business could be harmed. In addition, subject to certain exceptions, CoBiz has agreed to operate its business in the ordinary course prior to closing, and BOK is also subject to certain restrictions on the conduct of its business prior to closing. For more information, see “The Merger Agreement—Covenants and Agreements,” beginning on page 72.

Failure to complete the merger could negatively impact the stock prices, future businesses and financial results of BOK and CoBiz.

If the merger is not completed, the ongoing businesses of BOK and CoBiz may be adversely affected, and BOK and CoBiz will be subject to several risks, including the following:

In addition, if the merger is not completed, BOK and/or CoBiz may experience negative reactions from the financial markets and from their respective customers and employees. For example, BOK’s and CoBiz’s businesses may be impacted adversely by the failure to pursue other beneficial opportunities due to the focus of management on the merger, without realizing any of the anticipated benefits of completing the merger. The market price of BOK’s or CoBiz’s common stock could decline to the extent that the current market prices reflect a market assumption that the merger will be completed. BOK and/or CoBiz also could be subject to litigation related to any failure to complete the merger or to proceedings commenced against BOK or CoBiz to perform their respective obligations under the merger agreement. If the merger is not completed, BOK and CoBiz cannot assure their stockholders and shareholders, respectively, that the risks described above will not materialize and will not materially affect the business, financial results and stock prices of BOK and/or CoBiz.

If the merger is not completed, BOK and CoBiz will have incurred substantial expenses without realizing the expected benefits of the merger.

Each of BOK and CoBiz has incurred and will incur substantial expenses in connection with the negotiation and completion of the transactions contemplated by the merger agreement, as well as the costs and expenses of filing, printing and mailing this proxy statement/prospectus and all filing and other fees paid to the SEC in connection with the merger. If the merger is not completed, BOK and CoBiz would have to recognize these expenses without realizing the expected benefits of the merger.

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The shares of BOK common stock to be received by CoBiz’s shareholders as a result of the merger will have different rights from shares of CoBiz common stock.

Following completion of the merger, CoBiz’s shareholders will become stockholders of BOK, and their rights as stockholders will be governed by the Oklahoma General Corporation Act (which we refer to as the “OGCA”) and BOK’s charter and bylaws. There will be important differences between your current rights as a shareholder of CoBiz and the rights to which you will be entitled as a stockholder of BOK. For a discussion of the different rights associated with BOK common stock and CoBiz common stock, see “Comparison of Shareholder Rights,” beginning on page 88.

Risks Relating to the Tax Consequences of the Merger

If the merger does not constitute a reorganization under Section 368(a) of the Internal Revenue Code, then CoBiz shareholders may be responsible for payment of U.S. income taxes related to the consideration they receive in the merger.

The United States Internal Revenue Service (“IRS”) may determine that the merger does not qualify as a tax-free reorganization under Section 368(a) of the Internal Revenue Code of 1986, as amended (the “Code”). In that case, each CoBiz shareholder would recognize a gain or loss equal to the difference between the sum of any cash consideration and the fair market value of the shares of BOK common stock received by the shareholder in the merger, and such shareholder’s adjusted tax basis in the shares of CoBiz common stock exchanged therefor.

Risks Relating to BOK Business

You should read and consider risk factors specific to BOK’s business that will also affect the combined company after the merger. These risks are described in the sections entitled “Risk Factors” in BOK’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017 and in other documents incorporated by reference into this proxy statement/prospectus. For more information, see “Where You Can Find More Information,” beginning on page 102.

Risks Relating to CoBiz’s Business

You should read and consider risk factors specific to CoBiz’s business that will also affect the combined company after the merger. These risks are described in the sections entitled “Risk Factors” in CoBiz’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017 and in other documents incorporated by reference into this proxy statement/prospectus. For more information, see “Where You Can Find More Information,” beginning on page 102.

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INFORMATION REGARDING FORWARD-LOOKING STATEMENTS

This proxy statement/prospectus, including information included or incorporated by reference in this proxy statement/prospectus contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These forward-looking statements include, but are not limited to, statements about the benefits of the merger between CoBiz and BOK, including CoBiz and BOK future financial or business performance or conditions; statements about BOK’s and CoBiz’s plans, objectives, expectations and intentions with respect to future operations, products and services; and other statements identified by words such as “believe,” “expect,” “anticipate,” “intend,” “target,” “estimate,” “continue,” “positions,” “plan,” “predict,” “project,” “forecast,” “guidance,” “goal,” “objective,” “prospects,” “possible” or “potential,” by future conditional verbs such as “assume,” “will,” “would,” “should,” “could” or “may”, or by variations of such words or by similar expressions. These forward-looking statements are subject to numerous assumptions, risks and uncertainties, which change over time, are difficult to predict and are generally beyond the control of either company. Forward-looking statements speak only as of the date they are made, and BOK and CoBiz assume no duty to update forward-looking statements. Actual results may differ materially from current projections.

In addition to factors previously disclosed in CoBiz Financial Inc.’s and BOK Financial Corporation’s reports filed with the SEC and those identified elsewhere in this proxy statement/prospectus, the following factors, among others, could cause actual results to differ materially from forward-looking statements or historical performance:

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Additional factors that could cause BOK’s and CoBiz’s results to differ materially from those described in the forward-looking statements can be found in BOK’s and CoBiz’s filings with the SEC, including BOK’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017 and CoBiz’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017.

You are cautioned not to place undue reliance on the forward-looking statements, which speak only as of the date of this proxy statement/prospectus or the date of any document incorporated by reference in this proxy statement/prospectus. All subsequent written and oral forward-looking statements concerning the merger or other matters addressed in this proxy statement/prospectus and attributable to BOK or CoBiz or any person acting on their behalf are expressly qualified in their entirety by the cautionary statements contained or referred to in this section. Except to the extent required by applicable law or regulation, BOK and CoBiz undertake no obligation to update these forward-looking statements to reflect events or circumstances after the date of this proxy statement/prospectus or to reflect the occurrence of unanticipated events.

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THE SPECIAL MEETING OF COBIZ’S SHAREHOLDERS

This section contains information for CoBiz’s shareholders about the special meeting that CoBiz has called to allow its shareholders to consider and vote on the approval of the merger agreement and other matters. CoBiz is mailing this proxy statement/prospectus to you, as a shareholder of CoBiz, on or about [●], 2018. This proxy statement/prospectus is accompanied by a notice of the special meeting of CoBiz’s shareholders and a form of proxy card that CoBiz’s board of directors is soliciting for use at the special meeting and at any adjournments or postponements of the special meeting.

Date, Time and Place of the Special Meeting

The special meeting of CoBiz’s shareholders will be held at CoBiz’s office at 1401 Lawrence Street, Suite 1200, Denver, Colorado 80202 on September 27, 2018 at 8:00 a.m., local time.

Purpose of the Special Meeting

At the special meeting, CoBiz’s shareholders as of the record date will be asked to consider and vote on the following proposals:

Recommendation of CoBiz’s Board of Directors

CoBiz’s board of directors has unanimously approved and adopted the merger agreement and the transactions contemplated thereby and recommends that you vote “FOR” the merger proposal, “FOR” the compensation proposal and “FOR” the adjournment proposal.

Under the Colorado Business Corporation Act, if the merger is completed, holders of CoBiz common stock who do not vote in favor of approval of the merger agreement will have the right to seek appraisal of the fair value of their shares, but only if they submit a written demand for such an appraisal prior to the vote on the merger agreement and they comply with the other Colorado law procedures and requirements explained in this proxy statement/prospectus. A copy of Article 113 of the Colorado Business Corporation Act is attached to this proxy statement/prospectus as Annex C.

Record Date, Outstanding Shares, Shares Entitled to Vote and Quorum

CoBiz’s board of directors has fixed the close of business on August 14, 2018 as the record date for determining the holders of CoBiz common stock entitled to receive notice of and to vote at the CoBiz special meeting.

As of the record date, there were 42,397,697 shares of CoBiz common stock outstanding and entitled to vote at the CoBiz special meeting held by 362 holders of record. Each shareholder of record on that date is entitled to one vote for each share held on each proposal to be considered at the special meeting.

The presence (in person or by proxy) of holders of at least a majority of the shares entitled to vote at the special meeting constitutes a quorum for transacting business at the special meeting. All shares of CoBiz common stock present in person or represented by proxy, including abstentions and broker non-votes, will be treated as present and entitled to vote for purposes of determining the presence or absence of a quorum for all matters voted on at the special meeting.

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Vote Required, Treatment of Abstentions and Failure to Vote

Merger proposal:

Compensation proposal:

Adjournment proposal:

Shares Held by Officers and Directors

As of the record date, the directors and executive officers of CoBiz and their affiliates collectively owned, and were entitled to vote, 3,509,557 shares of CoBiz common stock, representing approximately 8.3% of CoBiz’s outstanding shares on that date. CoBiz currently expects that each of its directors and executive officers and their affiliates will vote their shares of CoBiz common stock “FOR” the approval of the merger agreement.

On June 17, 2018, BOK entered into Voting Agreements with each of Steven Bangert and Noel N. Rothman. As of August 14, 2018, Mr. Bangert and Mr. Rothman beneficially owned in the aggregate 3,124,816 shares of CoBiz common stock (representing approximately 7.4% of the outstanding shares of CoBiz common stock). Mr. Bangert and Mr. Rothman each agreed, on the terms and subject to the conditions set forth in the voting agreement, to vote his respective shares of CoBiz common stock in favor of the approval of the merger agreement and the approval of the transactions contemplated thereby, including the merger, and other related matters, and to vote against any acquisition proposal. The voting agreement will terminate on the earliest to occur of (i) the effective time of the merger, and (ii) the termination of the merger agreement. Copies of the voting agreements are attached to this proxy statement/prospectus as Annex B-1 and Annex B-2 and are incorporated by reference herein in their entirety. For more information, see “Voting Agreements,” beginning on page 69.

When considering CoBiz’s board of directors’ recommendation that you vote in favor of the approval of the merger agreement, you should be aware that certain of the executive officers and directors of CoBiz have interests in the merger that may be different from, or in addition to, the interests of CoBiz’s shareholders generally. For more information, see “The Merger—Interests of CoBiz’s Directors and Executive Officers in the Merger,” beginning on page 54.

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Voting of Proxies and Incomplete Proxies

A shareholder of CoBiz may vote by proxy or in person at the special meeting. If you hold your shares of CoBiz common stock in your name as a shareholder of record, to submit a proxy, you as a shareholder of CoBiz, may use one of the following methods:

CoBiz’s board of directors requests that you vote by telephone, over the Internet or by completing and signing the accompanying proxy card and returning it to CoBiz as soon as possible in the enclosed postage-paid envelope.

All properly signed proxies received prior to the special meeting and not revoked before the vote at the special meeting will be voted at the special meeting according to the instructions indicated on the proxies or, if no instructions are given, the shares will be voted “FOR” the merger proposal, “FOR” the compensation proposal and “FOR” the adjournment proposal.

Your vote is important. Whether or not you plan to attend the special meeting, please take the time to vote by completing and mailing the enclosed proxy card or by submitting a proxy through the Internet or by telephone as described on the enclosed instructions as soon as possible to make sure your shares are represented at the special meeting. Submitting a proxy now will NOT prevent you from being able to vote in person at the special meeting.

Shares Held in Street Name and Broker Non-Votes

If your shares are held in “street name,” meaning in the name of a bank, broker or other nominee who is the record holder, you must provide the record holder of your shares with instructions on how to vote the shares. Please follow the voting instructions provided by the bank, broker or other nominee. You may not vote shares held in street name by returning a proxy card directly to CoBiz or by voting in person at the CoBiz special meeting, unless you provide a “legal proxy,” which you must obtain from your bank, broker or other nominee. Further, brokers, banks or other nominees who hold shares of CoBiz common stock on behalf of their customers may not give a proxy to CoBiz to vote those shares with respect to any of the proposals without specific instructions from their customers, as brokers, banks and other nominees do not have discretionary voting power on these matters.

Failure to instruct your bank, broker or other nominee how to vote will have the same effect as a vote “AGAINST” approval of the merger agreement.

How to Revoke Your Proxy

If you are a holder of record of CoBiz common stock, you may revoke your proxy at any time by taking any of the following actions before your proxy is voted at the special meeting:

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You should send any notice of revocation or your completed new proxy card, as the case may be, to CoBiz at the following address:

CoBiz Financial Inc.

1401 Lawrence Street, Suite 1200

Denver, Colorado 80202

Attn: Richard J. Dalton, Corporate Secretary

If you choose to send a completed proxy card bearing a later date than your original proxy card, the new proxy card must be received before the beginning of the special meeting.

If you hold your shares in “street name” through a bank, broker or other nominee, you must contact your record holder to change your vote.

Voting in Person

If you are a shareholder of record of CoBiz on August 14, 2018, plan to attend the special meeting and wish to vote in person, you will be given a ballot at the special meeting. Please note, however, that if your shares are held of record by a bank, broker or other nominee and you wish to vote at the special meeting, you must obtain a “legal proxy,” executed in your favor, from the bank, broker or other nominee who is the holder of record of your shares, in order to vote your shares in person at the special meeting.

If you plan to attend the special meeting, you must hold your shares in your own name or have a letter from the record holder of your shares confirming your ownership. In addition, you must bring a form of personal photo identification with you in order to be admitted to the meeting. You must also bring your admission ticket with you to be admitted to the special meeting. CoBiz reserves the right to refuse admittance to anyone without proper proof of share ownership or without proper photo identification. Whether or not you intend to be present at the special meeting, you are urged to sign, date and return your proxy card or to vote via the Internet or by telephone, promptly. If you are then present and wish to vote your shares in person, your original proxy may be revoked by voting at the special meeting.

Whether or not you plan to attend the special meeting, CoBiz requests that you complete, sign, date and return the enclosed proxy card as soon as possible in the enclosed postage-paid envelope or submit a proxy through the Internet or by telephone as described in the instructions accompanying this proxy statement/prospectus. This will not prevent you from voting in person at the special meeting, but will assure that your vote is counted if you are unable to attend.

Proxy Solicitation

The enclosed proxy is solicited by and on behalf of CoBiz’s board of directors. CoBiz will pay the expenses of soliciting proxies to be voted at the special meeting, except that CoBiz and BOK have each agreed to share equally the costs of printing and mailing this document, other than attorneys’ and accountants’ fees, which will be paid by the party incurring the expense. Following the original mailing of the proxies and other soliciting materials, CoBiz and its agents also may solicit proxies by mail, telephone, facsimile, electronic mail or in person. No additional compensation will be paid to directors, officers or other employees of CoBiz for making these solicitations.

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CoBiz has retained a proxy solicitation firm, Mackenzie Partners, Inc., to aid it in the solicitation process. CoBiz will pay Mackenzie Partners, Inc. a fee of approximately $15,000 plus certain expenses. CoBiz intends to reimburse persons who hold CoBiz common stock of record but not beneficially, such as brokers, custodians, nominees and fiduciaries, for their reasonable expenses in forwarding copies of proxies and other soliciting materials to, and requesting authority for the exercise of proxies from, the persons for whom they hold the shares.

Stock Certificates

If you are a shareholder of CoBiz, you should not send in any certificates representing CoBiz common stock. Following the completion of the merger, you will receive separate instructions for the exchange of your certificates representing CoBiz common stock.

Delivery of Proxy Materials to Shareholders Sharing an Address

Only one proxy statement/prospectus is delivered to two or more of CoBiz’s shareholders who share an address unless CoBiz or its agent has received contrary instructions from one or more of the shareholders. CoBiz will deliver promptly upon written or oral request a separate copy of these documents to CoBiz’s shareholders at a shared address to which a single copy was delivered. To request that separate copies of these documents be delivered, CoBiz’s shareholders can contact CoBiz’s proxy solicitor by mail at 1407 Broadway, 27th Floor, New York, NY 10018 or by telephone at 1-800-322-2885. You may also contact CoBiz’s proxy solicitor if you received multiple copies of a proxy statement of CoBiz and would prefer to receive a single copy in the future.

Assistance

If you need assistance in completing your proxy card, have questions regarding CoBiz’s special meeting or would like additional copies of this proxy statement/prospectus, please contact Investor Relations at (303) 312-3412 or CoBiz’s proxy solicitor, Mackenzie Partners, Inc., at 1-800-322-2885.

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THE PROPOSALS

PROPOSAL NO. 1: MERGER PROPOSAL

CoBiz is asking its shareholders to approve the merger agreement. The holders of CoBiz common stock should read this proxy statement/prospectus carefully and in its entirety, including the annexes, for more detailed information concerning the merger agreement and the merger. A copy of the merger agreement is attached to this proxy statement/prospectus as Annex A.

After careful consideration, CoBiz’s board of directors, by a unanimous vote of all directors, determined that the merger, on the terms and conditions set forth in the merger agreement, is in the best interests of CoBiz and its shareholders. For a more detailed discussion of CoBiz’s board of directors’ recommendation, see “The Merger— CoBiz’s Reasons for the Merger; Recommendation of CoBiz’s Board of Directors,” beginning on page 43.

CoBiz’s board of directors unanimously recommends a vote “FOR” the merger proposal.

PROPOSAL NO. 2: COMPENSATION PROPOSAL

Pursuant to the Dodd-Frank Act and Rule 14a-21(c) of the Securities Exchange Act of 1934, as amended (which we refer to as the “Exchange Act”), CoBiz is seeking non-binding, advisory approval from its shareholders of the compensation of CoBiz’s named executive officers that is based on or otherwise relates to the merger, as disclosed in “The Merger—Interests of CoBiz’s Directors and Executive Officers in the Merger—Quantification of Potential Payments to CoBiz’s Named Executive Officers in Connection with the Merger,” beginning on page 54. The proposal gives CoBiz’s shareholders the opportunity to express their views on the merger-related compensation of CoBiz’s named executive officers. Accordingly, CoBiz is requesting its shareholders to approve the following resolution, on a non-binding, advisory basis:

“RESOLVED, that the compensation that may be paid or become payable to CoBiz’s named executive officers in connection with the merger, and the agreements or understandings pursuant to which such compensation may be paid or become payable, in each case as disclosed pursuant to Item 402(t) of Regulation S-K in “The Merger—Interests of CoBiz’s Directors and Executive Officers in the Merger—Quantification of Potential Payments to CoBiz’s Named Executive Officers in Connection with the Merger,” are hereby APPROVED on a non-binding, advisory basis.”

Approval of this proposal is not a condition to completion of the merger, the vote with respect to this proposal is advisory only and will not be binding on BOK or CoBiz and the vote on this proposal is separate and apart from the merger proposal. If the merger is completed, the merger-related compensation may be paid to CoBiz’s named executive officers to the extent payable in accordance with the terms of the compensation agreements and arrangements even if CoBiz’s shareholders fail to approve the advisory vote regarding merger-related compensation.

CoBiz’s board of directors unanimously recommends a vote “FOR” the compensation proposal.

PROPOSAL NO. 3: ADJOURNMENT PROPOSAL

The special meeting may be adjourned to another time or place, if necessary or appropriate, to permit, among other things, further solicitation of proxies if necessary to obtain additional votes in favor of the merger proposal.

If, at the special meeting, the number of shares of CoBiz common stock present or represented and voting in favor of the merger proposal is insufficient to approve such proposal, CoBiz intends to move to adjourn the special meeting in order to solicit additional proxies for the approval of the merger agreement. In this proposal, CoBiz is asking its shareholders to authorize the holder of any proxy solicited by CoBiz’s board of directors on a discretionary basis to vote in favor of adjourning the special meeting to another time and place for the purpose of soliciting additional proxies, including the solicitation of proxies from CoBiz’s shareholders who have previously voted.

CoBiz’s board of directors unanimously recommends a vote “FOR” the adjournment proposal.

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INFORMATION ABOUT THE COMPANIES

BOK Financial Corporation

BOK Financial Corporation

Boston Avenue at Second Street

Tulsa, Oklahoma 74192

(918) 588-6000

BOK was incorporated in Oklahoma in 1990 as a financial holding company governed by the Bank Holding Company Act of 1956, as amended, BOK offers full service banking in metropolitan areas of Oklahoma, Texas, New Mexico, Northwest Arkansas, Colorado, Arizona, and Kansas/Missouri. At March 31, 2018, BOK reported total consolidated assets of $33.4 billion. BOKF, NA is a wholly owned subsidiary bank of BOK. BOKF, NA operates TransFund, Cavanal Hill Investment Management, BOK Financial Asset Management, Inc. and seven banking divisions: Bank of Albuquerque, Bank of Arizona, Bank of Arkansas, Mobank, Bank of Oklahoma, Bank of Texas and Colorado State Bank and Trust. Other wholly owned subsidiaries of BOK include BOK Financial Securities, Inc., a broker/dealer that primarily engages in retail and institutional securities sales and municipal bond underwriting and The Milestone Group, Inc., an investment adviser to high net worth clients. As of December 31, 2017, BOK and its subsidiaries employed 4,930 full-time equivalent employees.

BOK operates three principal lines of business: Commercial Banking, Consumer Banking and Wealth Management. Commercial Banking includes lending, treasury and cash management services and customer risk management products for small businesses, middle market and larger commercial customers. Commercial Banking also includes the TransFund electronic funds network. Consumer Banking includes retail lending and deposit services, lending and deposit services to small business customers served through the retail branch network and all mortgage banking activities. Wealth Management provides fiduciary services, private bank services and investment advisory services in all markets. Wealth Management also underwrites state and municipal securities and engages in brokerage and trading activities.

BOKF Merger Sub Corporation Number Sixteen

BOKF Merger Corporation Number Sixteen

Boston Avenue at Second Street

Tulsa, Oklahoma 74192

(918) 588-6000

Merger Sub, a wholly owned subsidiary of BOK, was incorporated in Oklahoma in 2018 for the sole purpose of effecting the merger.

CoBiz Financial Inc.

CoBiz Financial Inc.

1401 Lawrence Street, Suite 1200

Denver, Co 80202

Phone (303) 312-3400

CoBiz was incorporated in Colorado in 1980 and is a financial services company that serves the complete financial needs of businesses, business owners and professionals in Colorado and Arizona. The company’s subsidiaries are: CoBiz Bank, which provides commercial banking services; CoBiz IM, Inc. which offers investment management and wealth planning through CoBiz Wealth; and CoBiz Insurance, Inc. which delivers employee benefits, and property and casualty insurance brokerage. As of March 31, 2018, CoBiz had total assets of $3.8 billion. CoBiz had 538 full-time equivalent employees as of December 31, 2017.

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THE MERGER

The following is a discussion of the merger and the material terms of the merger agreement between BOK and CoBiz. You are urged to read carefully the merger agreement in its entirety, a copy of which is attached as Annex A to this proxy statement/prospectus and incorporated by reference herein. This summary does not purport to be complete and may not contain all of the information about the merger agreement that is important to you. We encourage you to read the merger agreement carefully and in its entirety. This section is not intended to provide you with any factual information about BOK or CoBiz. Such information can be found elsewhere in this proxy statement/prospectus and in the public filings BOK and CoBiz make with the SEC. See “Where You Can Find More Information” in the forepart of this proxy statement/prospectus.

Terms of the Merger

Transaction Structure

BOK’s board of directors has approved and CoBiz’s board of directors has approved the merger agreement and the transactions contemplated thereby. The merger agreement provides for the merger of CoBiz with and into Merger Sub, with Merger Sub continuing as the surviving corporation. Immediately following the merger or at such later time as BOK may determine, CoBiz Bank, a Colorado state-chartered bank and a wholly-owned subsidiary of CoBiz, will merge with and into BOKF, National Association, a national banking association and a wholly-owned subsidiary of BOK, with BOKF continuing as the surviving entity.

Before the completion of the merger, BOK may change the method or structure of effecting the combination of BOK and CoBiz, provided, that no such change may (1) alter or change the merger consideration or the number of shares of BOK common stock received by CoBiz’s shareholders in exchange for each share of CoBiz common stock, (2) adversely affect the tax treatment of CoBiz’s or BOK’s shareholders, (3) adversely affect the tax treatment of BOK or CoBiz or (4) materially impede or delay the closing of the transactions contemplated by the merger agreement in a timely manner.

Merger Consideration

If the merger is completed, each share of CoBiz common stock issued and outstanding immediately prior to the effective time of the merger (other than dissenting shares or shares of common stock owned by CoBiz, BOK or Merger Sub), will be converted into the right to receive 0.17 shares of BOK common stock and $5.70 in cash.

BOK will not issue any fractional shares of BOK common stock in the merger. Instead, a CoBiz shareholder who otherwise would have received a fraction of a share of BOK common stock will receive an amount in cash (rounded to the nearest cent) determined by multiplying the fraction of the BOK common stock to which the holder would otherwise be entitled by the average closing sale prices of BOK common stock on NASDAQ as reported by the Wall Street Journal for the five full trading days ending on the trading day prior to the closing date of the merger.

Conversion of Shares; Exchange and Payment Procedures

At or prior to the closing, BOK will deposit or cause to be deposited with an exchange agent designated by BOK and reasonably acceptable to CoBiz, for the benefit of the holders of shares of CoBiz common stock, sufficient cash and BOK common stock to be exchanged in accordance with the merger agreement, including the payment of cash in lieu of fractional shares.

The conversion of CoBiz common stock into the right to receive the merger consideration will occur automatically at the effective time of the merger. After completion of the merger, the exchange agent will exchange any certificates representing shares of CoBiz common stock for the merger consideration to be received pursuant to the terms of the merger agreement.

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Letters of Transmittal

As promptly as practicable after the effective time of the merger, and in any event within 5 days thereafter, the exchange agent will mail to each holder of record of CoBiz common stock immediately prior to the effective time of the merger a letter of transmittal and instructions on how to surrender shares of CoBiz common stock in exchange for the merger consideration and any cash in lieu of fractional shares the holder is entitled to receive under the merger agreement.

If a certificate for CoBiz common stock has been lost, stolen, or destroyed, the exchange agent will issue the merger consideration upon receipt of (1) an affidavit of that fact by the claimant and (2) if required by BOK, the posting of a bond in an amount as BOK may determine is reasonably necessary as indemnity against any claim that may be made against it with respect to such certificate.

After completion of the merger, there will be no further transfers on the stock transfer books of CoBiz of shares of CoBiz common stock that were issued and outstanding immediately prior to the effective time

Withholding

BOK and the exchange agent will be entitled to deduct and withhold from any cash portion of the merger consideration or any cash in lieu of fractional shares, cash dividends or distributions payable, or any other amounts otherwise payable pursuant to the merger agreement to any holder of CoBiz common stock or CoBiz equity awards the amounts they are required to deduct and withhold under the Code or any provision of state, local, or foreign tax law. If any such amounts are withheld and paid over to the appropriate governmental authority, these amounts will be treated for all purposes of the merger agreement as having been paid to the holder of CoBiz common stock or CoBiz equity awards from whom they were withheld.

Dividends and Distributions

No dividends or other distributions declared with respect to BOK common stock will be paid to the holder of any unsurrendered certificates of CoBiz common stock until the holder surrenders such certificate in accordance with the merger agreement. After the surrender of a certificate in accordance with the merger agreement, the record holder thereof will be entitled to receive any such dividends or other distributions, without any interest, which had previously become payable with respect to the whole shares of BOK common stock that the shares of CoBiz common stock represented by such certificate have been converted into the right to receive under the merger agreement.

Background of the Merger

As part of its efforts to enhance long-term value for its shareholders, the CoBiz Board of Directors has periodically reviewed and evaluated the company’s strategic direction and business objectives. Since its founding in 1980, the CoBiz board has concluded that the best way to build long-term shareholder value has been through continued investment in the company’s franchise as an independent company.

In early March of this year, Steven Bangert, CoBiz’s Chairman and Chief Executive Officer, was approached by the chief executive officer of a larger banking organization (which we refer to as “Company A”), who indicated an interest in pursuing a business combination of CoBiz with Company A, potentially as part of a multistep process to develop a larger presence in Colorado. Mr. Bangert indicated to Company A’s chief executive officer that while the CoBiz Board of Directors was not currently considering any sale or similar business combination transaction, he would discuss Company A’s interest with the CoBiz board at an upcoming board meeting.

On March 14, 2018, Company A and CoBiz entered into a confidentiality agreement.

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On March 16, 2018, Company A sent Mr. Bangert a letter outlining proposed terms for a transaction in which each share of CoBiz common stock would be exchanged for Company A common shares valued at $22.44 per CoBiz share based on Company A’s stock trading price at that time. The proposal also contemplated that Mr. Bangert would join Company A’s board at closing and also serve as chief executive officer for Company A’s Colorado operations.

On March 19, 2018, the CoBiz Board of Directors met. Representatives from BofA Merrill Lynch, CoBiz’s financial advisor, and Simpson Thacher & Bartlett LLP (which we refer to as “Simpson Thacher”) and Sherman & Howard L.L.C. (which we refer to as “Sherman & Howard”), CoBiz’s legal advisors, attended the meeting. Mr. Bangert informed the CoBiz board of his meeting with the chief executive officer of Company A and Company A’s proposal for a business combination with CoBiz. The CoBiz board reviewed and discussed Company A’s proposal as well as CoBiz’s historical financial performance and future prospects. The CoBiz board also discussed Company A’s proposal in the context of precedent bank merger transactions and the operating environment and challenges faced by regional banks. A representative of BofA Merrill Lynch then provided a financial review of a potential transaction with Company A and an overview of Company A’s past acquisitions, valuation and the evolution of its franchise. A representative of Simpson Thacher reviewed with the board its fiduciary duties. The CoBiz board then discussed in detail possible strategic alternatives to enhance long-term shareholder value for CoBiz’s shareholders, including a review of possible other acquirers that might be interested in a transaction with CoBiz as well as the potential benefits and risks of remaining independent. The review of potential acquirers included a discussion of the potential acquirers’ financial capacity to engage in a transaction and execution risk, among other factors. During this discussion, it was noted that BOK might be interested in a potential transaction with CoBiz, based on general expressions of interest over the past several years by its chief executive officer in discussing a potential business combination (which expressions of interest did not include any price terms). The board also noted that BOK had the financial resources and geographic overlap to be able to offer an attractive price with a stock that appeared to have significant long-term upside. Following these discussions, the CoBiz board determined that CoBiz’s management and advisors should continue their valuation analysis of CoBiz, and also continue exploratory discussions with Company A as well as contacting BOK to ascertain its level of interest in pursuing business combination discussions at this time.

On March 20, 2018, executives of Company A met with Mr. Bangert and other executives of CoBiz. The executives discussed their respective companies and operations. The executives also reviewed the strategic benefits of a combination.

On March 23, 2018, Mr. Bangert called Steven Bradshaw, the chief executive officer of BOK, to inquire about his interest in pursuing business combination discussions. Mr. Bradshaw indicated that expanding in Colorado was a high priority for BOK and that based on their familiarity with CoBiz and its franchise, BOK was very interested in pursuing business combination discussions. Mr. Bradshaw indicated that he would work with BOK’s management team to provide an initial proposal within the next several weeks.

On March 26, 2018, the CoBiz Board of Directors met. Representatives from BofA Merrill Lynch, Simpson Thacher and Sherman & Howard attended the meeting. The CoBiz board continued its discussion of Company A’s proposal, including a review of the potential strengths and risks of a potential transaction with Company A. Mr. Bangert reported on the results of the meeting between executives of CoBiz and Company A, noting that CoBiz’s management had concerns regarding the compatibility of the two companies’ operating models and the absence of significant synergies and cost savings.

Representatives from BofA Merrill Lynch reviewed the results of its preliminary valuation analysis of CoBiz and provided an updated transaction analysis of Company A’s proposal, both stand-alone and pro forma for possible Colorado expansion, and an analysis of the trading history of Company A’s stock and business model. Following this presentation, the CoBiz board continued its discussion of other potential merger partners, including BOK and the consideration mix BOK could propose. Additionally, the CoBiz board discussed the significant synergies and cost-savings that could potentially be realized from a transaction with BOK, and the long-term value this could create for CoBiz’s shareholders. Following this discussion, it was the consensus of the CoBiz board that of the

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potential merger partners BOK had the best strategic fit and was the most likely to be able to provide long-term value to CoBiz’s shareholders at relatively low risk, and that it was therefore in the best interests of CoBiz and its shareholders to pursue further business combination discussions with BOK. Following a discussion, the CoBiz board expanded the Capital Formation Committee to four members, consisting of Ms. Rhinehart and Messrs. Bangert, Rothman and Hutchinson, and delegated to the Committee the authority to oversee discussions with potential merger partners between meetings of the full CoBiz board.

On April 2, 2018, BOK submitted a preliminary indication of interest. The proposal provided for a fixed exchange ratio of 0.2185x to 0.2285x (valued at $21.63 to $22.62 per CoBiz share based on BOK’s March 29, 2018 share trading price), with the option for up to $500 million of this consideration to be payable in cash depending upon CoBiz’s objectives. The proposal also indicated that CoBiz would potentially receive representation on BOK’s board of directors.

On April 19, 2018, the Capital Formation Committee of the CoBiz Board of Directors met. Representatives from BofA Merrill Lynch, Simpson Thacher and Sherman & Howard attended the meeting. Mr. Bangert and the BofA Merrill Lynch representatives provided an update to the Committee on the value of the Company A proposal based on share trading prices of CoBiz and Company A over the past month. The Committee discussed in detail Company A’s growth over the past years, including its significant acquisitions, and the potential risks in its loan portfolio and potential share price volatility.    

Representatives from BofA Merrill Lynch then reviewed with the Committee the financial aspects of BOK’s preliminary indication of interest and certain financial metrics, as well as a detailed overview of the BOK franchise. Following this presentation, the Committee discussed various strategic benefits to a transaction with BOK compared to a transaction with Company A, including the highly aligned business models and cultures of CoBiz and BOK. The Committee also noted that BOK had a strong record of delivering long-term value to its shareholders, with a total shareholder return that had consistently outperformed its peer group, and noted that its growth to date had been based primarily on internal investment and organic expansion. Following a discussion, it was the consensus of the Committee to recommend that the CoBiz board authorize management to engage in negotiations with BOK.

On April 26, 2018, the CoBiz Board of Directors met. Representatives from BofA Merrill Lynch, Simpson Thacher and Sherman & Howard attended the meeting. The CoBiz board discussed BOK’s and Company A’s proposals, including a review of the operations of both companies. Representatives from BofA Merrill Lynch reviewed the discussions that had occurred with BOK to date. The BofA Merrill Lynch representatives then provided an overview of macroeconomic issues and the implications for the banking industry in general. Following an overview of the banking landscape, the BofA Merrill Lynch representatives reviewed its preliminary valuation analysis of CoBiz and provided an updated transaction analysis of BOK’s proposal. After a full discussion, it was the consensus of the CoBiz board that the BOK proposal appeared to provide superior long-term value compared to Company A’s proposal, based on the conversion ratios and consideration mix offered as well as the more compatible operating model, greater potential synergies and cost savings, and BOK’s history of strong and consistent organic growth. The board also confirmed its earlier determination that BOK had the best strategic fit and potential for long-term shareholder value at relatively low execution risk compared to the other potential merger partners the board considered. Accordingly, the CoBiz board instructed senior management to continue pursuing discussions with BOK.

On April 30, 2018, CoBiz and BOK entered into a non-disclosure agreement. Shortly thereafter, CoBiz granted representatives of BOK access to a document data room, and from that date to June 17, 2018 BOK conducted due diligence, which included in person meetings and telephonic communications with CoBiz. During this time CoBiz also performed reverse due diligence on BOK.

On May 15, 2018, Mr. Bangert and Mr. Bradshaw spoke to discuss the status of BOK’s due diligence review of CoBiz and related matters. During this call Mr. Bradshaw reaffirmed BOK’s strategic interest in a business

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combination with CoBiz and reconfirmed his views of his originally proposed exchange ratio range. He also requested additional diligence materials to use in refining BOK’s valuation models.

Over the next several weeks management of CoBiz and representatives of BofA Merrill Lynch spoke with their counterparts at BOK and its financial advisors to discuss various valuation and due diligence matters, and during these conversations they emphasized the importance of BOK’s final offer being at the upper end of BOK’s initial proposal range.

On May 24, 2018, the Capital Formation Committee of the CoBiz Board of Directors held a meeting to receive an update on the discussions and due diligence review process with BOK.

On June 1, 2018, BOK sent a letter of intent to CoBiz proposing a merger of CoBiz into a subsidiary of BOK, in which each outstanding share of CoBiz common stock would be exchanged for 0.17 shares of BOK common stock and $5.70 in cash, representing a price range of $22.18 - $23.59 per share of CoBiz common stock based on the closing price of BOK common stock from March 29^th (the date of BOK’s initial proposal letter) through June 1^st.

Later that day, the Capital Formation Committee met. Representatives from BofA Merrill Lynch, Simpson Thacher and Sherman & Howard attended the meeting. Mr. Bangert began the meeting by providing an overview of BOK’s proposal. Representatives of BofA Merrill Lynch then provided a financial review of BOK’s proposal and various financial metrics, and noted that BOK’s proposal was at the higher end of the preliminary pricing range submitted by BOK on March 29^th. After a discussion of other potential benefits and risks of a transaction with BOK, the Committee authorized senior management of CoBiz to negotiate the letter of intent with BOK with the view of presenting it to the full board for consideration in the near future.

On June 2, 2018, CoBiz provided a draft of the merger agreement prepared by Simpson Thacher to Frederic Dorwart, Lawyers, PLLC (which we refer to as “Frederic Dorwart”), BOK’s legal advisors. From that point through June 17, 2018, Simpson Thacher and Frederic Dorwart exchanged multiple revised drafts of the definitive agreement and participated in discussions and negotiations with respect to the drafts.    

On June 5, 2018, the CoBiz Board of Directors met. Representatives from BofA Merrill Lynch, Simpson Thacher and Sherman & Howard attended the meeting. Mr. Bangert reviewed the terms of BOK’s proposal. Representatives of BofA Merrill Lynch then provided a financial review of BOK’s proposal and various financial metrics, including a comparison to precedent transactions. The board reviewed again the potential benefits and challenges of remaining independent, including the need for significant additional investments in technology and management resources. After a full discussion of other potential benefits and risks of a transaction with BOK, the CoBiz board instructed senior management to execute the letter of intent with BOK and continue negotiations with BOK.

Over the next several weeks Mr. Bangert and Mr. Bradshaw spoke several times to discuss, among other things, strategic and operational matters, and the managements and advisors to both companies continued discussions on due diligence and other matters.

On June 6, 2018, CoBiz provided BOK with drafts of the voting agreements to be entered into by Mr. Bangert and Mr. Rothman to Frederic Dorwart.

On June 11, 2018, the Capital Formation Committee met. Representatives of BofA Merrill Lynch, Simpson Thacher and Sherman & Howard attended the meeting. The Committee discussed the status of the transaction negotiations with BOK. The representative of Simpson Thacher reviewed the current version of the merger agreement with the board. Representatives of BofA Merrill Lynch then provided a market update of BOK’s and CoBiz’s share trading prices and ratios and reviewed certain financial metrics.

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On June 15, 2018, the CoBiz Board of Directors met. Representatives of BofA Merrill Lynch, Simpson Thacher and Sherman & Howard attended the meeting. Mr. Bangert began the meeting by providing an update to the board of recent developments relating to the potential transaction with BOK. A representative of Simpson Thacher reviewed the current version of the merger agreement with the board, including the changes that had been made from the draft previously discussed. The representatives of BofA Merrill Lynch provided an update on the transaction negotiations with BOK. BofA Merrill Lynch then presented materials summarizing its financial analyses of BOK’s proposal. After a review of the BofA Merrill Lynch materials and a detailed discussion of other potential benefits and risks of a transaction with BOK, the CoBiz board determined that the proposed transaction with BOK was in the best long-term interests of CoBiz’s shareholders and approved the merger agreement and authorized management to execute the merger agreement, subject to final approval by the Capital Formation Committee and delivery of BofA Merrill Lynch’s fairness opinion.

From June 15^th through June 17^th management of CoBiz and BOK and their respective advisors negotiated the final terms of the merger agreement and related matters.

On June 17, 2018, the Capital Formation Committee of the CoBiz Board of Directors held a meeting to consider and approve the final terms of the proposed transaction with BOK. Representatives of BofA Merrill Lynch, Simpson Thacher and Sherman & Howard attended the meeting. A representative of Simpson Thacher reviewed the execution version of the definitive merger agreement with the Committee. Representatives of BofA Merrill Lynch then delivered its written opinion to the effect that as of June 17, 2018, and subject to the conditions, limitations and qualifications stated therein, the merger consideration provided for in the merger agreement was fair to holders of CoBiz common stock from a financial point of view. The Committee then unanimously approved the merger agreement, the merger and the other transactions contemplated by the merger agreement, and authorized management to execute and deliver the merger agreement. That evening the merger agreement and the ancillary agreements were executed by the parties.

On June 18, 2018, CoBiz and BOK issued a joint press release before the opening of stock markets in New York announcing the execution of the merger agreement.

Recommendation of the CoBiz Board of Directors and Reasons for the Merger

In reaching its decision to approve the merger agreement, the merger and the other transactions contemplated by the merger agreement, and to recommend that its shareholders approve merger agreement, the CoBiz Board of Directors evaluated the merger agreement, the merger and the other transactions contemplated by the merger agreement in consultation with CoBiz management, as well as CoBiz’s financial and legal advisors, and considered a number of factors, including the following:

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The foregoing discussion of the information and factors considered by the CoBiz Board of Directors is not intended to be exhaustive, but includes the material factors considered by the CoBiz Board of Directors. In reaching its decision to approve the merger agreement, the merger and the other transactions contemplated by the merger agreement, the CoBiz Board of Directors did not quantify or assign any relative weights to the factors considered, and individual directors may have given different weights to different factors. The CoBiz Board of Directors considered all these factors as a whole, including discussions with CoBiz’s management and CoBiz’s financial and legal advisors, and overall considered the factors to be favorable to, and to support, its determination.

The foregoing discussion of the information and factors considered by the CoBiz Board of Directors is forward-looking in nature. This information should be read in light of the factors described under the section entitled “Cautionary Statement Regarding Forward-Looking Statements.”

For the reasons set forth above, the CoBiz Board of Directors determined that the merger agreement and the transactions contemplated by the merger agreement are in the best interests of CoBiz and its shareholders, and approved the merger agreement and the transactions contemplated thereby. The CoBiz Board of Directors recommends that holders of CoBiz common stock vote “FOR” the approval of the merger agreement, “FOR” the merger-related compensation proposal and “FOR” the common shareholder adjournment proposal (if necessary or appropriate).

Certain Unaudited Prospective Financial Information

Neither CoBiz nor BOK makes, as a matter of course, public disclosure of forecasts or projections of its expected financial performance because of, among other things, the inherent difficulty of accurately predicting financial performance for future periods and the inherent uncertainty of the underlying assumptions. However, CoBiz management provided BofA Merrill Lynch with certain nonpublic unaudited prospective financial information for 2018–2023 prepared by CoBiz management and BOK management provided BofA Merrill Lynch with certain nonpublic unaudited prospective financial information for 2018-2020, that was utilized by BofA Merrill Lynch for purposes of financial analyses performed in connection with its fairness opinion, as described under “—Opinion of CoBiz’s Financial Advisor” beginning on page 47. CoBiz management also provided BOK with the same nonpublic unaudited prospective financial information for 2018 and 2019 that was prepared by CoBiz management. A summary of certain significant elements of this prospective financial information is set forth below and is included in this proxy statement/prospectus solely because such information was made available to BofA Merrill Lynch in connection with its fairness opinion and BOK in connection with their due diligence investigation of CoBiz.

The prospective financial information for CoBiz and BOK reflects numerous estimates and assumptions with respect to industry performance, general business, economic, regulatory, market and financial conditions and other future events, as well as matters specific to CoBiz’s and BOK’s respective businesses, all of which are inherently uncertain and difficult to predict and many of which are beyond CoBiz’s and BOK’s control. The prospective financial information is subjective in many respects and thus is susceptible to multiple interpretations and periodic revisions based on actual experience and business developments. The prospective financial information may also be affected by CoBiz’s and BOK’s ability to achieve strategic goals, objectives and targets over the applicable periods. As such, the prospective financial information constitutes forward-looking information and is subject to risks and uncertainties, including the risks and uncertainties described under “Risk Factors,” beginning on page 22, “Cautionary Statement Regarding Forward-Looking Statements,” beginning on page 29, and in CoBiz’s Annual Report on Form 10-K for the year ended December 31, 2017 and BOK’s Annual Report on Form 10-K for the year ended December 31, 2017 and the other reports filed by each of CoBiz and

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BOK with the SEC that are incorporated by reference into this proxy statement/prospectus, as described under “Where You Can Find More Information,” beginning on page 102.

The prospective financial information for CoBiz and BOK was generally not prepared with a view toward public disclosure or complying with GAAP, the published guidelines of the SEC regarding forward-looking statements or the guidelines established by the American Institute of Certified Public Accountants for preparation and presentation of prospective financial information. Neither CoBiz’s nor BOK’s independent registered public accounting firm, nor any other independent accountants, have compiled, examined or performed any procedures with respect to the prospective financial information included below, nor have they expressed any opinion or any other form of assurance on such information or its achievability, and they assume no responsibility for, and disclaim any association with, the prospective financial information. Furthermore, the prospective financial information does not take into account any circumstances or events occurring after the date it was prepared.

You are strongly cautioned not to place undue reliance on the prospective financial information set forth below. The inclusion of the prospective financial information in this proxy statement/prospectus should not be regarded as an indication that any of CoBiz, BOK or their respective affiliates, advisors or representatives considered or consider the prospective financial information to be necessarily predictive of actual future events, and the prospective financial information should not be relied upon as such. None of CoBiz, BOK or their respective affiliates, advisors or representatives can give any assurance that actual results will not differ from the prospective financial information, and none of them undertakes any obligation to update or otherwise revise or reconcile the prospective financial information to reflect circumstances existing after the date such information was prepared or to reflect the occurrence of future events even in the event that any or all of the assumptions underlying the prospective financial information are shown to be in error or to reflect changes in general economic or industry conditions. None of CoBiz, BOK or their respective affiliates, advisors or representatives makes any representation to any shareholder regarding the projections.

In light of the foregoing, and considering that the special meeting will be held several months after the prospective financial information was prepared, as well as the uncertainties inherent in any forecasted information, CoBiz’s shareholder are cautioned not to place unwarranted reliance on such information, and BOK and CoBiz urge all shareholders to review CoBiz’s and BOK’s most recent SEC filings for a description of CoBiz’s and BOK’s reported financial results. See “Where You Can Find More Information” in the forepart of this proxy statement/prospectus beginning on page 102.

The prospective financial information summarized in this section is not being included in this proxy statement/prospectus in order to induce any CoBiz shareholder to vote in favor of any of the proposals to be voted on at the special meeting.

The following table presents summary selected unaudited prospective financial information of CoBiz prepared by CoBiz’s management that was provided to BofA Merrill Lynch and, solely with respect to 2018 and 2019, BOK.

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The following table presents summary selected unaudited prospective financial information of BOK prepared by BOK’s management.

Opinion of CoBiz’s Financial Advisor

CoBiz has retained BofA Merrill Lynch to act as CoBiz’s financial advisor in connection with the merger. BofA Merrill Lynch is an internationally recognized investment banking firm which is regularly engaged in the valuation of businesses and securities in connection with mergers and acquisitions, negotiated underwritings, secondary distributions of listed and unlisted securities, private placements and valuations for corporate and other purposes. CoBiz selected BofA Merrill Lynch to act as CoBiz’s financial advisor in connection with the merger on the basis of BofA Merrill Lynch’s experience in transactions similar to the merger, its reputation in the investment community and its familiarity with CoBiz and its business.

On June 15, 2018, at a meeting of CoBiz’s board of directors held to evaluate the merger, BofA Merrill Lynch delivered to CoBiz’s board of directors an oral opinion, which was confirmed by delivery of a written opinion dated June 17, 2018, to the effect that, as of the date of the opinion and based on and subject to various assumptions and limitations described in its opinion, the merger consideration to be received by holders of CoBiz common stock was fair, from a financial point of view, to such holders.

The full text of BofA Merrill Lynch’s written opinion to CoBiz’s board of directors, which describes, among other things, the assumptions made, procedures followed, factors considered and limitations on the review undertaken, is attached as Annex D to this document and is incorporated by reference herein in its entirety. The following summary of BofA Merrill Lynch’s opinion is qualified in its entirety by reference

to the full text of the opinion. BofA Merrill Lynch delivered its opinion to CoBiz’s board of directors for the benefit and use of CoBiz’s board of directors (in its capacity as such) in connection with and for purposes of its evaluation of the merger consideration from a financial point of view. BofA Merrill Lynch’s opinion does not address any other aspect of the merger and no opinion or view was expressed as to the relative merits of the merger in comparison to other strategies or transactions that might be available to CoBiz or in which CoBiz might engage or as to the underlying business decision of CoBiz to proceed with or effect the merger. BofA Merrill Lynch’s opinion does not address any other aspect of the merger and does not constitute a recommendation to any shareholder as to how to vote or act in connection with the proposed merger or any other matter.

In connection with rendering its opinion, BofA Merrill Lynch has, among other things:

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In arriving at its opinion, BofA Merrill Lynch assumed and relied upon, without independent verification, the accuracy and completeness of the financial and other information and data publicly available or provided to or otherwise reviewed by or discussed with it and relied upon the assurances of the managements of CoBiz and BOK that they were not aware of any facts or circumstances that would make such information or data inaccurate or misleading in any material respect. With respect to the CoBiz management forecasts, BofA Merrill Lynch was advised by CoBiz, and assumed, that they were reasonably prepared on bases reflecting the best currently available estimates and good faith judgments of the management of CoBiz as to the future financial performance of CoBiz. With respect to the BOK management forecasts and cost savings, BofA Merrill Lynch was advised by BOK, and assumed, with CoBiz’s consent, that they were reasonably prepared on bases reflecting the best currently available estimates and good faith judgments of the management of BOK as to the future financial performance of BOK and other matters covered thereby. BofA Merrill Lynch relied, at the direction of CoBiz, on the assessments of the managements of CoBiz and BOK as to BOK’s ability to achieve the cost savings and was advised by BOK, and assumed, with the consent of CoBiz, that the cost savings will be realized in the amounts and at the times projected. BofA Merrill Lynch relied, at CoBiz’s direction, upon the assessments of the management of CoBiz as to the potential impact of market, governmental and regulatory trends and developments relating to or affecting CoBiz and its business. BofA Merrill Lynch did not make and was not provided with any independent evaluation or appraisal of the assets or liabilities (contingent or otherwise) of CoBiz or BOK, nor did it make any physical inspection of the properties or assets of CoBiz or BOK. BofA Merrill Lynch did not evaluate the solvency or fair value of CoBiz or BOK under any state, federal or other laws relating to bankruptcy, insolvency or similar matters. BofA Merrill Lynch assumed, at the direction of CoBiz, that the merger would be consummated in accordance with its terms, without waiver, modification or amendment of any material term, condition or agreement and that, in the course of obtaining the necessary governmental, regulatory and other approvals, consents, releases and waivers for the merger, no delay, limitation, restriction or

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condition, including any divestiture requirements or amendments or modifications, would be imposed that would have an adverse effect on CoBiz, BOK or the contemplated benefits of the merger. BofA Merrill Lynch also assumed, at the direction of CoBiz, that the final executed merger agreement would not differ in any material respect from the draft merger agreement reviewed by it.

BofA Merrill Lynch expressed no opinion or view as to any terms or other aspects or implications of the merger (other than the merger consideration to the extent expressly specified in its opinion), including, without limitation, the form or structure of the merger, any related transactions or any other agreement, arrangement or understanding entered into in connection with or related to the merger or otherwise. BofA Merrill Lynch was not requested to, and it did not, solicit indications of interest or proposals from third parties regarding a possible acquisition of all or any part of CoBiz or any alternative transaction. BofA Merrill Lynch’s opinion was limited to the fairness, from a financial point of view, of the merger consideration to be received by the holders of CoBiz common stock and no opinion or view was expressed with respect to any consideration received in connection with the merger by the holders of any other class of securities, creditors or other constituencies of any party. In addition, no opinion or view was expressed with respect to the fairness (financial or otherwise) of the amount, nature or any other aspect of any compensation to any of the officers, directors or employees of any party to the merger, or class of such persons, relative to the merger consideration or otherwise. Furthermore, no opinion or view was expressed as to the relative merits of the merger in comparison to other strategies or transactions that might be available to CoBiz or in which CoBiz might engage or as to the underlying business decision of CoBiz to proceed with or effect the merger. BofA Merrill Lynch did not express any opinion or view as to what the value of BOK common stock actually would be when issued or the prices at which CoBiz common stock or BOK common stock would trade at any time, including following announcement or consummation of the merger. BofA Merrill Lynch also did not express any opinion or view with respect to, and BofA Merrill Lynch relied, at the direction of CoBiz, upon the assessments of representatives of CoBiz regarding, legal, regulatory, accounting, tax and similar matters relating to CoBiz or the merger, as to which matters BofA Merrill Lynch understood that CoBiz obtained such advice as it deemed necessary from qualified professionals. In addition, BofA Merrill Lynch expressed no opinion or recommendation as to how any shareholder should vote or act in connection with the merger or any other matter.

BofA Merrill Lynch’s opinion was necessarily based on financial, economic, monetary, market and other conditions and circumstances as in effect on, and the information made available to BofA Merrill Lynch as of, the date of its opinion. It should be understood that subsequent developments may affect its opinion, and BofA Merrill Lynch does not have any obligation to update, revise or reaffirm its opinion. The issuance of BofA Merrill Lynch’s opinion was approved by a fairness opinion review committee of BofA Merrill Lynch. Except as described in this summary, CoBiz imposed no other limitations on the investigations made or procedures followed by BofA Merrill Lynch in rendering its opinion.

The discussion set forth below in the section entitled “Summary of Material Financial Analyses” represents a brief summary of the material financial analyses presented by BofA Merrill Lynch to CoBiz’s board of directors in connection with its opinion. The financial analyses summarized below include information presented in tabular format. In order to fully understand the financial analyses performed by BofA Merrill Lynch, the tables must be read together with the text of each summary. The tables alone do not constitute a complete description of the financial analyses performed by BofA Merrill Lynch. Considering the data set forth in the tables below without considering the full narrative description of the financial analyses, including the methodologies and assumptions underlying the analyses, could create a misleading or incomplete view of the financial analyses performed by BofA Merrill Lynch. For purposes of the financial analyses described below, the value of the merger consideration is deemed to be $23.06 per share of CoBiz common stock, calculated as the sum of (i) the cash consideration of $5.70 per share of CoBiz common stock and (ii) the implied value of the stock consideration of $17.36 per share of CoBiz common stock based on the closing per share price of BOK common stock of $102.09 on June 14, 2018 and the exchange ratio of 0.17 share of BOK common stock per share of CoBiz common stock.

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Summary of Material Financial Analyses.

Selected Publicly Traded Companies Analysis. BofA Merrill Lynch reviewed publicly available financial and stock market information for CoBiz and the following eleven U.S. west coast-based publicly traded banks and thrifts with assets between $3 billion and $10 billion, which excluded companies traded over-the-counter and pending merger targets:

BofA Merrill Lynch reviewed, among other things, per share equity values, based on closing stock prices on June 14, 2018, of the selected publicly traded companies as a multiple of tangible book value per share, commonly referred to as TBVPS, as of March 31, 2018, and calendar year 2018 and 2019 estimated earnings per share, commonly referred to as EPS. The overall low to high TBVPS multiples observed for selected publicly traded companies were 1.71x to 3.43x (with a median of 2.34x). The overall low to high calendar year 2018 estimated EPS multiples observed for selected publicly traded companies were 13.6x to 21.5x (with a median of 17.3x). The overall low to high calendar year 2019 estimated EPS multiples observed for selected publicly traded companies were 11.7x to 20.0x (with a median of 14.0x). BofA Merrill Lynch then applied ranges of TBVPS multiples as of March 31, 2018 and calendar year 2018 and 2019 EPS multiples of 1.99x to 2.70x, 14.7x to 19.9x and 11.9x to 16.1x, respectively, derived from the selected publicly traded companies to CoBiz’s estimated TBVPS as of March 31, 2018 and calendar year 2018 and 2019 estimated EPS.

BofA Merrill Lynch also reviewed core deposit premiums of the selected publicly traded companies, calculated as market capitalization less tangible common equity, as a percentage of core deposits (representing total deposits other than time deposits of greater than $250,000, in each case as of March 31, 2018. The overall low to high core deposit premiums observed for selected publicly traded companies were 7.5% to 33.1% (with a median of 20.6%) of core deposits. BofA Merrill Lynch then applied ranges of core deposit premiums of 17.5% to 23.7% derived from the selected publicly traded companies to CoBiz’s core deposits as of March 31, 2018.

Estimated financial data of the selected publicly traded companies were based on publicly available research analysts’ estimates, and estimated financial data of CoBiz were based on the CoBiz management forecasts. This analysis indicated the following approximate implied per share equity value reference ranges for CoBiz, as compared to the merger consideration:

No company used in this analysis is identical or directly comparable to CoBiz. Accordingly, an evaluation of the results of this analysis is not entirely mathematical. Rather, this analysis involves complex considerations and

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judgments concerning differences in financial and operating characteristics and other factors that could affect the public trading or other values of the companies to which CoBiz was compared.

Selected Precedent Transactions Analysis. BofA Merrill Lynch reviewed, to the extent publicly available, financial information relating to the following seventeen selected U.S. bank merger transactions with a value of greater than $500 million announced since January 1, 2017:

BofA Merrill Lynch reviewed per share transaction values, calculated as the enterprise value implied for the target company based on the consideration payable in the selected transaction, as a multiple of the target company’s TBVPS as of March 31, 2018. The overall low to high multiples of the target companies’ TBVPS for the selected transactions were 1.56x to 3.19x (with a median of 2.41x). BofA Merrill Lynch then applied TBVPS multiples of 2.05x to 2.77x derived from the selected transactions to CoBiz’s estimated TBVPS as of March 31, 2018. Estimated financial data of the selected transactions were based on publicly available information. Estimated financial data of CoBiz were based on the CoBiz management forecasts. This analysis indicated the following approximate implied per share equity value reference range for CoBiz, as compared to the merger consideration:

No company, business or transaction used in this analysis is identical or directly comparable to CoBiz or the merger. Accordingly, an evaluation of the results of this analysis is not entirely mathematical. Rather, this analysis involves complex considerations and judgments concerning differences in financial and operating characteristics and other factors that could affect the acquisition or other values of the companies, business segments or transactions to which CoBiz and the merger were compared.

Discounted Cash Flow Analysis. BofA Merrill Lynch performed a discounted cash flow analysis of CoBiz to calculate the estimated present value of the capital available for distribution that CoBiz was forecasted to generate during 2018 through 2023 based on forecasts provided by CoBiz management. For purposes of calculating the terminal value, BofA Merrill Lynch utilized 2024 net income as approved by CoBiz management. BofA Merrill Lynch calculated terminal values for CoBiz by applying terminal forward multiples of (i) 12.0x to 16.0x to CoBiz’s fiscal year 2024 estimated EPS and (ii) 2.00x to 2.75x to CoBiz’s estimated TBVPS as of December 31, 2023. The capital available for distribution and terminal values were then discounted to present

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value as of December 31, 2018 using discount rates ranging from 8.6% to 10.8%, which were based on an estimate of CoBiz’s weighted average cost of capital. This analysis indicated the following approximate implied per share equity value reference ranges for CoBiz as compared to the merger consideration:

Other Factors.

BofA Merrill Lynch also noted certain additional factors that were not considered part of BofA Merrill Lynch’s material financial analyses with respect to its opinion but were referenced for informational purposes, including, among other things, the following:

Miscellaneous.

As noted above, the discussion set forth above in the sections entitled “Summary of Material Financial Analyses” is a summary of the material financial analyses presented by BofA Merrill Lynch to CoBiz’s board of directors in connection with its opinion and is not a comprehensive description of all analyses undertaken or factors considered by BofA Merrill Lynch in connection with its opinion. The preparation of a financial opinion is a complex analytical process involving various determinations as to the most appropriate and relevant methods of financial analysis and the application of those methods to the particular circumstances and, therefore, a financial opinion is not readily susceptible to partial analysis or summary description. BofA Merrill Lynch believes that its analyses summarized above must be considered as a whole. BofA Merrill Lynch further believes that selecting portions of its analyses and the factors considered or focusing on information presented in tabular format, without considering all analyses and factors or the narrative description of the analyses, could create a misleading or incomplete view of the processes underlying BofA Merrill Lynch’s analyses and opinion. The fact that any specific analysis has been referred to in the summary above is not meant to indicate that such analysis was given greater weight than any other analysis referred to in the summary.

In performing its analyses, BofA Merrill Lynch considered industry performance, general business and economic conditions and other matters, many of which are beyond the control of CoBiz and BOK. The estimates of the

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future performance of CoBiz and BOK in or underlying BofA Merrill Lynch’s analyses are not necessarily indicative of actual values or actual future results, which may be significantly more or less favorable than those estimates or those suggested by BofA Merrill Lynch’s analyses. These analyses were prepared solely as part of BofA Merrill Lynch’s analysis of the fairness, from a financial point of view, to the holders of CoBiz common stock of the merger consideration to be received by such holders and were provided to CoBiz’s board of directors in connection with the delivery of BofA Merrill Lynch’s opinion. The analyses do not purport to be appraisals or to reflect the prices at which a company might actually be sold or the prices at which any securities have traded or may trade at any time in the future. Accordingly, the estimates used in, and the ranges of valuations resulting from, any particular analysis described above are inherently subject to substantial uncertainty and should not be taken to be BofA Merrill Lynch’s view of the actual values of CoBiz or BOK.

The type and amount of consideration payable in the merger was determined through negotiations between CoBiz and BOK, rather than by any financial advisor, and was approved by CoBiz’s board of directors. The decision to enter into the merger agreement was solely that of CoBiz’s board of directors. As described above, BofA Merrill Lynch’s opinion and analyses were only one of many factors considered by CoBiz’s board of directors in its evaluation of the proposed merger and should not be viewed as determinative of the views of CoBiz’s board of directors or management with respect to the merger or the merger consideration.

CoBiz has agreed to pay BofA Merrill Lynch for its services in connection with the merger an aggregate fee currently estimated to be approximately $9.8 million, $1 million of which was payable upon delivery of its opinion and the remaining portion of which is contingent upon consummation of the merger.    CoBiz also has agreed to reimburse BofA Merrill Lynch for its expenses incurred in connection with BofA Merrill Lynch’s engagement and to indemnify BofA Merrill Lynch, any controlling person of BofA Merrill Lynch and each of their respective directors, officers, employees, agents and affiliates against specified liabilities, including liabilities under the federal securities laws.

BofA Merrill Lynch and its affiliates comprise a full service securities firm and commercial bank engaged in securities, commodities and derivatives trading, foreign exchange and other brokerage activities, and principal investing as well as providing investment, corporate and private banking, asset and investment management, financing and financial advisory services and other commercial services and products to a wide range of companies, governments and individuals. In the ordinary course of their businesses, BofA Merrill Lynch and its affiliates may invest on a principal basis or on behalf of customers or manage funds that invest, make or hold long or short positions, finance positions or trade or otherwise effect transactions in equity, debt or other securities or financial instruments (including derivatives, bank loans or other obligations) of CoBiz, BOK and certain of their respective affiliates.

BofA Merrill Lynch and its affiliates in the past have provided, currently are providing, and in the future may provide investment banking, commercial banking and other financial services to CoBiz and have received or in the future may receive compensation for the rendering of these services, including having provided or providing certain derivatives trading services to CoBiz. From June 1, 2016 through May 31, 2018, BofA Merrill Lynch and its affiliates derived aggregate revenues from CoBiz of less than $1 million for investment and corporate banking services.

In addition, BofA Merrill Lynch and its affiliates in the past have provided, currently are providing, and in the future may provide investment banking, commercial banking and other financial services to BOK and have received or in the future may receive compensation for the rendering of these services, including (i) having acted as joint book-running manager and underwriter on a debt offering for BOK, (ii) having acted or acting as lender or under certain leasing and other credit facilities for BOK, (iii) having provided or providing certain treasury management services and products to BOK and (iv) having provided or providing certain derivatives trading services to BOK. From June 1, 2016 through May 31, 2018, BofA Merrill Lynch and its affiliates derived aggregate revenues from BOK of approximately $5 million for investment and corporate banking services.

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BOK’s Board of Directors’ Reasons for the Merger

BOK reasons for entering into the merger agreement include the opportunity to:

The Executive Committee, acting with full authority of the Board of Directors, unanimously approved the merger agreement after executive management discussed the above as well as the terms of the merger agreement, CoBiz employees and management teams, results of due diligence, reputation of CoBiz, assets, liabilities, results of operations and risk factors. In view of the variety of factors considered in connection with the evaluation of the merger, the Executive Committee did not find it practicable to, and did not, quantify or otherwise assign relative weights to the specific factors considered in reaching its determination and recommendation. The Executive Committee based its position on all the information and factors presented to and considered by it. In addition, individual directors may have given differing weights to different factors.

This explanation of the Executive Committee’s reasoning and all other information in this section is forward-looking and therefore should be read in conjunction with the factors discussed under the heading “Information Regarding Forward Looking Statements” beginning on page 29. The members of the Executive Committee include the Chairman of the Board, the Chief Executive Officer and the Chairpersons of each of the committees of the Board of Directors.

Management and Board of Directors of BOK After the Merger

Pursuant to the merger agreement, BOK has agreed to appoint to its and BOKF, National Association’s Boards of Directors, at or prior to the effective time of the merger, Steven Bangert or, if Mr. Bangert is unable to be appointed, such other member of the Board of Directors of CoBiz mutually agreed upon by CoBiz and BOK. BOK agreed to include Mr. Bangert in the slate of directors at the annual meeting of BOK’s stockholders to occur after the effective time of the merger. Information regarding current directors of BOK and CoBiz, including biographical information, compensation and stock ownership, can be found in each of BOK’s and CoBiz’s proxy statements for their respective 2018 annual meetings of shareholders, which are filed with the SEC and incorporated by reference into this proxy statement/prospectus. See “Where You Can Find More Information” in the forepart of this proxy statement/prospectus.

Interests of CoBiz Directors and Executive Officers in the Merger

In considering the recommendation of CoBiz’s board of directors with respect to the merger, CoBiz’s shareholders should be aware that the directors and executive officers of CoBiz have certain interests in the merger that may be different from, or in addition to, the interests of CoBiz’s shareholders generally. CoBiz’s board of directors was aware of these interests and considered them, among other matters, in making its recommendation that CoBiz’s shareholders vote to approve the merger proposal. These interests are described in further detail below.

Conversion of Shares of CoBiz Common Stock Held by Directors and Officers of CoBiz Pursuant to the Merger

As a group, the CoBiz directors and executive officers beneficially owned, as of July 31, 2018, an aggregate of approximately 3,509,557 shares of CoBiz common stock. At the effective time of the merger, each share of CoBiz common stock held by the directors and executive officers that is issued and outstanding immediately prior to the effective time of the merger will, like all other shares of CoBiz common stock, be converted into the

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right to receive (i) 0.17 shares of BOK common stock, and (ii) $5.70 in cash, for each share of CoBiz common stock. Dispositions of shares of CoBiz common stock, if any, prior to the closing of the merger will change the number of shares of BOK common stock and cash the directors and executive officers of CoBiz will receive in respect of their shares of CoBiz common stock upon the closing of the merger.

The following table sets forth the total number of shares of CoBiz common stock beneficially owned by each CoBiz executive officer and director, as of July 31, 2018.

Conversion of CoBiz Stock Options

The CoBiz executive officers held, as of July 31, 2018, options to acquire 1,150 shares of CoBiz common stock, at a weighted average exercise price of $12.18. The CoBiz non-employee directors held, as of July 31, 2018, options to acquire 30,183 shares of CoBiz common stock, at a weighted average exercise price of $10.63.

At the effective time of the merger, each outstanding option to acquire shares of CoBiz common stock held by executive officers and directors will vest (to the extent unvested) and be cancelled and converted automatically into the right to receive the merger consideration with respect to a number of shares of CoBiz common stock equal to (x) the product of (A) the number of shares of CoBiz common stock subject to such stock option multiplied by (B) the excess, if any, of (i) the sum of the per share stock consideration and the per share cash consideration over (ii) the exercise price per share, divided by (y) the sum of the per share stock consideration and the per share cash consideration. The “per share stock consideration” means the exchange ratio of 0.17 times the average of the closing-sale prices of BOK common stock for the five full trading days ending on the trading day immediately preceding the effective time of the merger. The “per share cash consideration” means $5.70 in cash. Any stock option that has an exercise price per share that is greater than or equal to the sum of the per share stock consideration plus the per share cash consideration will be cancelled in exchange for no consideration.

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CoBiz may take actions during the 2018 calendar year to accelerate the vesting and settlement of the stock options to a date no later than December 31, 2018 in order to mitigate the impact of Sections 280G and 4999 of the Code, subject to approval of BOK.

The following table sets forth the number and weighted average exercise price of the vested and unvested stock options held by each executive officer and director of CoBiz, as of July 31, 2018.

Conversion of CoBiz Restricted Stock Awards

The executive officers held, as of July 31, 2018, 109,023 restricted stock awards.

At the effective time of the merger, each outstanding restricted stock award will vest (to the extent unvested) and be cancelled and converted automatically into the right to receive merger consideration with respect to the number of shares of CoBiz common stock subject to such restricted stock award. CoBiz may take actions during the 2018 calendar year to accelerate the vesting and settlement of the restricted stock awards to a date no later than December 31, 2018 in order to mitigate the impact of Sections 280G and 4999 of the Code, subject to approval of BOK.

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The following table sets forth the total number of restricted stock awards held by each executive officer and director of CoBiz, as of July 31, 2018.

Cash-Based Retention Programs

Under the merger agreement, CoBiz may establish a cash-based retention program for its employees (including the executive officers) to promote retention and incentivize efforts to consummate the closing, payable immediately prior to the effective time of the merger or upon an earlier qualifying termination of employment. Allocations of amounts under the closing date retention program are to be determined by CoBiz’s chief executive officer (or his designees), subject to the approval of BOK. If a retention award or portion thereof is forfeited by a participant prior to the closing date, CoBiz’s chief executive officer (or his designees) may reallocate the retention award (or unpaid portion thereof) to existing employees or new hires of CoBiz, subject to approval of BOK.

Under the merger agreement, CoBiz may also establish a cash-based retention program for its employees (including the executive officers) to promote retention and incentivize efforts through the CoBiz Bank conversion date. Participants (which may include the executive officers) and amounts under the conversion date retention program are to be mutually agreed between CoBiz and BOK.

Annual Bonus

Under the merger agreement, BOK is required to honor the bonus plans or arrangements in place for continuing CoBiz employees for the fiscal year in which closing occurs. To the extent closing occurs prior to the payment of bonuses in respect of any fiscal year of CoBiz, BOK will pay the annual bonuses to such employees in respect of such fiscal year in the ordinary course and based on actual performance through the end of the fiscal year (with any subjective performance criteria deemed achieved at 100% of target levels), subject to continued employment through the bonus payment date; provided that (i) if a continuing CoBiz employee is terminated without cause following the end of the fiscal year but prior to the bonus payment date, the employee is entitled to receive the full annual bonus amount, payable on the employee’s termination date and (ii) if a CoBiz employee is terminated immediately prior to closing or a continuing CoBiz employee is terminated prior to the end of the fiscal year in which closing occurs, the employee will be entitled to receive the annual bonus, prorated through the employee’s termination date, which bonus will be based on actual performance through the termination date (with any subjective performance criteria deemed achieved at 100% of target levels), payable on the termination date. All annual bonuses payable pursuant to the above will be payable in full in cash, including with respect to any portion of the bonus that would otherwise be payable in equity-based awards.

In addition, CoBiz may take actions during the 2018 calendar year to accelerate all or a portion of the bonus for the 2018 fiscal year to a date no later than December 31, 2018 in order to mitigate the impact of Sections 280G and 4999 of the Code, subject to the approval of BOK.

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Employment Agreements

CoBiz is party to employment agreements with each of its executive officers, which provide severance benefits upon a qualifying termination of employment. All such severance benefits are subject to the executive officer’s execution of a release of all claims against CoBiz and its affiliates.

If the employment of Messrs. Bangert, Dalton, Ostertag, or Pass or Ms. Andrich were terminated by the executive for any reason or by CoBiz without cause, in each case, within two years following a change in control of CoBiz (such as the merger), the executive officer would be entitled to the following:

If the employment of Messrs. Dumlao, Huss or Lindner or Ms. Hermann is terminated without cause or for good reason within six months (12 months for Mr. Dumlao) following a change in control of CoBiz (such as the merger), the executive officer would be entitled to the following:

For an estimate of the amounts that would become payable to CoBiz’s named executive officers under their employment agreements if each experienced a qualifying termination of employment immediately following the effective time of the merger, see “—Quantification of Potential Payments to CoBiz’s Named Executive Officers in Connection with the Merger” below. CoBiz estimates that the value of the compensation and benefits that would become payable to its four other executive officers under their change of control employment

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agreements if the effective time of the merger were July 31, 2018 and each experienced a severance-qualifying termination of employment immediately thereafter is $1.8 million in the aggregate.

Post-Closing Roles

Under the merger agreement, at or prior to the effective time of the merger, Mr. Bangert, or if Mr. Bangert is unable to be appointed, such other person mutually agreed upon by CoBiz and BOK, will be appointed to the board of directors of BOK and BOKF, National Association.

In addition, Mr. Bangert has been offered the role of Vice Chairman – Colorado of BOKF NA following the effective time of the merger. Mr. Bangert has entered into an employment agreement, which is effective upon and subject to the closing of the merger, with BOKF NA, a wholly owned subsidiary of BOK, CoBiz Bank, a wholly owned subsidiary of CoBiz, and for certain limited purposes, CoBiz. Pursuant to the employment agreement, Mr. Bangert will serve as Vice Chairman – Colorado of BOKF NA following the effective time of the merger. The employment agreement has an initial two-year term. During Mr. Bangert’s employment and for two years thereafter, he will be subject to certain customary non-solicitation and non-competition covenants. The employment agreement provides Mr. Bangert with an annual base salary of $50,000. Mr. Bangert will also receive a bonus payment within 3 days following the effective time of the merger equal to the payment he would have received under his prior employment agreement had he timely terminated such agreement within 24 months following a change in control as described above. In the event that Mr. Bangert is terminated without cause or is constructively discharged, Mr. Bangert is entitled to severance equal to the standard severance pay for senior banking employees at BOK plus a lump sum payment equal to his then annual salary.    Following the effective time of the merger, Mr. Bangert’s prior employment agreement shall be terminated and of no further force or effect.

Indemnification; Directors’ and Officers’ Insurance

The merger agreement provides that from and after the effective time of the merger, BOK will indemnify and hold harmless, to the fullest extent permitted by applicable law, each present and former director, officer or employee of CoBiz and its subsidiaries (in each case, when acting in such capacity) against any costs or expenses (including reasonable attorneys’ fees), judgments, fines, losses, damages or liabilities incurred in connection with any threatened or actual claim, action, suit, proceeding or investigation, whether arising before or after the effective time of the merger, arising in whole or in part out of, or pertaining to, the fact that such person is or was a director, officer or employee of CoBiz or its subsidiaries, and pertaining to matters, acts or omissions existing or occurring at or prior to the effective time of the merger, and will also advance expenses to such persons to the fullest extent permitted by applicable law, provided that such person provides an undertaking (in a reasonable and customary form) to repay such advances if it is ultimately determined that such person is not entitled to indemnification. In addition, for a period of six years from the effective time, BOK will maintain a liability insurance policy for the benefit of certain persons, including CoBiz’s directors and executive officers.

Quantification of Potential Payments to CoBiz’s Named Executive Officers in Connection with the Merger

The information set forth in the table below is intended to comply with Item 402(t) of Regulation S-K, which requires disclosure of information about certain compensation for each of CoBiz’s named executive officers (as identified in accordance with SEC regulations) that is based on or otherwise relates to the merger and assumes, among other things, that CoBiz’s named executive officers will incur a severance-qualifying termination of employment immediately following the effective time of the merger. For additional details regarding the terms of the payments described below, see the discussion under the caption “The Merger—Interests of CoBiz’s Directors and Executive Officers in the Merger.”

The amounts indicated below are estimates based on multiple assumptions that may or may not actually occur or be accurate on the relevant date, including assumptions described below, and do not reflect certain compensation

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actions that may occur before the effective time of the merger. For purposes of calculating such amounts, we have assumed: