UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): December 4, 2014

Kodiak Oil & Gas Corp.

(Exact name of registrant as specified in its charter)

1700 Broadway, Suite 2300, Denver, Colorado 80290-2300

(Address of principal executive offices, including ZIP code)

(303) 837-1661

(Registrant’s telephone number, including area code)

1625 Broadway, Suite 250, Denver, Colorado 80202

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On December 8, 2014, Kodiak Oil & Gas (USA) Inc. (“Kodiak USA”) terminated its Amended and Restated Credit Agreement, dated as of October 28, 2011, among Kodiak USA, Wells Fargo Bank, N.A. and the lenders party thereto, as amended (the “Kodiak Credit Agreement”). In connection with the termination of the Kodiak Credit Agreement, the related guarantee and pledge agreements were also terminated.

On December 8, 2014, pursuant to the Arrangement Agreement, dated as of July 13, 2014 (the “Agreement”), by and among Whiting Petroleum Corporation (“Whiting”), 1007695 B.C. Ltd., a wholly-owned subsidiary of Whiting (“Acquiror Canadian Sub”), and Kodiak Oil & Gas Corp. (“Kodiak”), Acquiror Canadian Sub acquired all of the outstanding shares of common stock, without par value, of Kodiak, and Acquiror Canadian Sub and Kodiak amalgamated with Kodiak surviving the amalgamation as a wholly-owned subsidiary of Whiting as part of a plan of arrangement (the “Arrangement”).

Pursuant to the Agreement, each outstanding share of Kodiak common stock was automatically converted at the effective time of the Arrangement into 0.177 shares of Whiting common stock, $0.001 par value per share (“Whiting Common Stock”). No fractional shares of Whiting Common Stock were issued. Where the aggregate number of shares of Whiting Common Stock issued to a Kodiak shareholder as consideration would have resulted in a fraction of shares of Whiting Common Stock being issued, the number of shares of Whiting Common Stock received by such Kodiak shareholder was rounded down to the nearest whole share of Whiting Common Stock. As a result of the Arrangement, Whiting issued approximately 47,500,000 shares of Whiting Common Stock. Kodiak stock options, restricted stock units and restricted stock awards automatically converted at the effective time of the Arrangement into 673,235 stock options, 257,601 restricted stock units and 47,325 restricted stock awards with respect to Whiting Common Stock, after giving effect to the 0.177 exchange ratio.

The foregoing description of the Agreement and the Arrangement does not purport to be complete and is qualified in its entirety by reference to the Agreement, a copy of which is filed as Exhibit 2.1 to this Current Report on Form 8-K and is incorporated herein by reference. There are representations and warranties contained in the Agreement, which were made by the parties to each other as of specific dates. The assertions embodied in these representations and warranties were made solely for purposes of the Agreement and may be subject to important qualifications and limitations agreed to by the parties in connection with negotiating their terms. Moreover, certain representations and warranties may not be accurate or complete as of any specified date because they are subject to a contractual standard of materiality that is different from certain standards generally applicable to stockholders or were used for the purpose of allocating risk between the parties rather than establishing matters as facts. Based upon the foregoing reasons, investors should not rely on the representations and warranties as statements of factual information.

On August 27, 2014, Whiting and Whiting Oil and Gas entered into a Sixth Amended and Restated Credit Agreement with the lenders party thereto, JPMorgan Chase Bank, N.A., as Administrative Agent, and the various other agents party thereto (the “Whiting Credit Agreement”). On December 8, 2014, Whiting, Whiting Oil and Gas Corporation, a wholly owned subsidiary of Whiting (“Whiting Oil and Gas”), Whiting US Holding Company, a wholly-owned subsidiary of Whiting (“Whiting US Holding”), Kodiak, Kodiak USA, Kodiak Williston, LLC and JPMorgan Chase Bank, N.A., as Administrative Agent, entered into an Amended and Restated Guaranty and

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Collateral Agreement under which Whiting US Holding, Kodiak, Kodiak USA and Kodiak Williston, LLC guaranteed the obligations under the Whiting Credit Agreement and pledged certain assets to secure the obligations thereunder (the “Guaranty and Collateral Agreement”).

As of December 8, 2014, the Whiting Credit Agreement became effective and replaced the previous credit agreement of Whiting Oil and Gas. Also on such date, Whiting Oil and Gas borrowed $1.5 billion under the Whiting Credit Agreement primarily to repay amounts outstanding under the Kodiak Credit Agreement, Whiting Oil and Gas’s previous credit agreements and other expenses due in connection with the completion of the Arrangement, leaving approximately $2.0 billion of available borrowing capacity as of December 8, 2014.

The foregoing descriptions of the Whiting Credit Agreement and Guaranty and Collateral Agreement do not purport to be complete and are qualified in their entirety by reference to the Whiting Credit Agreement and Guaranty and Collateral Agreement, copies of which are filed as Exhibit 4.1 and Exhibit 4.2, respectively, to this Current Report on Form 8-K and are incorporated herein by reference.

Kodiak First Supplemental Indentures

As disclosed in the Current Report on Form 8-K filed by Kodiak on October 20, 2014, on October 17, 2014, Whiting, Kodiak, certain Kodiak subsidiary guarantors, U.S. Bank National Association, as trustee, and Computershare Trust Company of Canada, as Canadian trustee, entered into supplemental indentures (the “First Supplemental Indentures”) to each indenture, as then supplemented (each, a “Kodiak Indenture”), under which Kodiak’s outstanding $800,000,000 aggregate principal amount of 8.125% Senior Notes Due 2019 (the “2019 Notes”), $350,000,000 aggregate principal amount of 5.500% Senior Notes Due 2021 (the “2021 Notes”) and $400,000,000 aggregate principal amount of 5.500% Senior Notes Due 2022 (the “2022 Notes” and, together with the 2019 Notes and 2021 Notes, the “Kodiak Notes”) were issued that provided for certain amendments to each Kodiak Indenture and the issuance of an unconditional and irrevocable guarantee by Whiting (the “Whiting Guarantee”) of the prompt payment, when due, of any amount owed to the holders of the Kodiak Notes under the Kodiak Notes and the Kodiak Indentures and any other amounts due pursuant to the Kodiak Indentures.

The amendments to the First Supplemental Indentures became operative and the Whiting Guarantee was issued upon the completion of the Arrangement (as such term is defined below).

The foregoing description of the First Supplemental Indentures does not purport to be complete and is qualified in its entirety by reference to the First Supplemental Indentures, copies of which are filed as Exhibit 4.3, Exhibit 4.4 and Exhibit 4.5 to this Current Report on Form 8-K and are incorporated herein by reference.

Kodiak Second Supplemental Indentures

On December 4, 2014, Whiting Oil and Gas, Whiting US Holding, Kodiak, certain Kodiak subsidiary guarantors, U.S. Bank National Association, as trustee, and Computershare Trust Company of Canada, as Canadian trustee, entered into supplemental indentures (the “Second Supplemental Indentures”) to the Kodiak Indentures under which the Kodiak Notes were issued that provided for the issuance of an unconditional and irrevocable guarantee by Whiting Oil and Gas (the “Whiting Oil and Gas Guarantee”) of the prompt payment, when due, of any amount owed to the holders of the Kodiak Notes under the Kodiak Notes and the Kodiak Indentures and any other amounts due pursuant to the Kodiak Indentures and Whiting US Holding to become a co-issuer of the Kodiak Notes.

The Whiting Oil and Gas Guarantee was issued on the completion of the Arrangement. Whiting US Holding will become a co-issuer upon the conversion of Kodiak into a British Columbia unlimited liability company, which is expected to occur shortly after completion of the Arrangement.

The foregoing description of the Second Supplemental Indentures does not purport to be complete and is qualified in its entirety by reference to the Second Supplemental Indentures, copies of which are filed as Exhibit 4.6, Exhibit 4.7 and Exhibit 4.8 to this Current Report on Form 8-K and are incorporated herein by reference.

                The consummation of the Arrangement on December 8, 2014, constituted a “change of control” under the Kodiak Indentures. As a result, each holder of outstanding Kodiak Notes may require Kodiak to make an offer (or such offer may, at Kodiak’s option, be made by a third party in lieu of Kodiak) within 30 days following the completion of the Arrangement to repurchase such notes for cash at 101% of the principal amount, plus accrued and unpaid interest, no earlier than 30 days or later than 60 days after such offer is made. Pursuant to the Kodiak Indentures, Kodiak (or, at Kodiak’s option, a third party in lieu of Kodiak) intends to mail a notice to each holder of the Kodiak Notes within 30 days of completion of the Arrangement that will include an offer to repurchase the Kodiak Notes.

In connection with the consummation of the Arrangement, the New York Stock Exchange (“NYSE”) was notified that each outstanding share of Kodiak’s common stock was converted into the right to receive 0.177 shares of Whiting Common Stock as described above, subject to the terms and conditions of the Agreement. Kodiak requested that the NYSE file a notification of removal from listing on Form 25 with the Securities and Exchange Commission (the “Commission”) with respect to delisting Kodiak’s shares of common stock. The shares of Kodiak common stock, which previously traded under the ticker symbol “KOG” on the NYSE, ceased trading on the NYSE on December 8, 2014. Kodiak intends to file with the Commission a Form 15 with respect to Kodiak common stock and the Kodiak Notes, requesting that the Kodiak common stock and Kodiak Notes be deregistered under the Securities Exchange Act of 1934.

The information disclosed under Item 2.01 is incorporated into this Item 3.03 in its entirety.

At the effective time of the Arrangement, holders of Kodiak’s common stock ceased to have any rights as shareholders of Kodiak, other than the right to receive consideration pursuant to the terms of the Arrangement.

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The information disclosed under Items 2.01 and 5.02 is incorporated into this Item 5.01 in its entirety.

As of the effective time of the Arrangement, Lynn A. Peterson and James E. Catlin were appointed to the board of directors of Whiting for terms to expire at Whiting’s annual meetings in 2017 and 2016, respectively, and until their successors are duly elected and qualified.

At the effective time of the Arrangement, (i) each executive officer and each member of Kodiak’s board of directors ceased to be executive officers and directors of Kodiak and (ii) James J. Volker and Michael J. Stevens were appointed to each serve as directors of Kodiak (the “New Directors”). The New Directors, at the effective time of the Arrangement, appointed James J. Volker to be the Chairman, President and Chief Executive Officer of Kodiak (principal executive officer), Michael J. Stevens to be the Vice President and Chief Financial Officer (principal financial officer) and Brent P. Jensen to be the Controller and Treasurer (chief accounting officer). Information regarding the backgrounds of Mr. Volker, Mr. Stevens and Mr. Jensen and other relevant information regarding Mr. Volker, Mr. Stevens and Mr. Jensen, is described in Whiting’s Annual Report on Form 10-K for the year ended December 31, 2013, which description is incorporated herein by reference.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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KODIAK OIL & GAS CORP.

FORM 8-K

EXHIBIT INDEX

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