Filed by Whiting Petroleum Corporation

(Commission File No. 001-31899)

Pursuant to Rule 425 of the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

of the Securities Exchange Act of 1934

Subject Company: Whiting Petroleum Corporation

(Commission File No. 001-31899)

Email to Whiting’s Employees

Whiting and Oasis Teams,

Consistent with our commitment to provide regular and transparent communications, we are writing with additional updates on the integration planning process.

Progress and timeline

Since our last communication, we have made the necessary filings with the Federal Trade Commission (FTC) and Antitrust Division of the Department of Justice (DOJ) under the HSR Act. This is part of the regulatory approval process related to our pending transaction.

We selected Boston Consulting Group (BCG) to be our external advisor on integration. BCG brings tremendous integration experience. They will work closely with the internal integration teams over the coming weeks and months.

The integration is expected to take approximately three to four months and is composed of three phases: Setup, Design & Planning, and Execution. The merger is expected to close in the second half of 2022.

Guiding principles for integration

To kick off the Setup phase of our integration planning, we have established guiding principles for our integration. All integration decisions will be guided by the following principles: Execution Certainty, Value Capture, and Organization, People, & Culture. Our integration team will anchor decisions in these guiding principles, and they will also be important cultural elements of our new company. There are a number of components to each of these principles, which we encourage you to read here.

At their core, these integration principles recognize the talent and operational excellence of both companies and seek to ensure that as we bring our companies together, we will maintain the strengths of each company.

Reminders

· Send us company name ideas by Friday! Thank you to everyone who has already submitted ideas for our new company name. If you have an idea you have not yet shared, send it to NewName@whiting.com (Whiting employees) or NewName@oasispetroleum.com (Oasis employees) by Friday close of business.

· We are working to get answers to your merger and integration questions. Please know that we have read the questions that you have submitted and are working to get you answers. We are still early in this process and many decisions have not yet been made. We intend to include Q&As in future integration planning updates. In the meantime, you can continue to submit questions to MergerQuestions@oasispetroleum.com (Oasis employees) and MergerQuestions@whiting.com (Whiting employees).

· Business As Usual: Until the close of the transaction, we are operating as separate companies. Please stay focused on executing safely and with integrity. For legal reasons, we want to remind you not to comment on social media about the transaction and to refrain from connecting with our future colleagues at the other company unless you are directed to do so in connection with the integration planning process.

Thank you for your continued hard work and dedication to working safely.

Sincerely,

Lynn Peterson and Danny Brown

1 Copyright © 2021 by Boston Consulting Group. All rights reserved. Guiding principles for the Integration process • Use agile decision - making to maintain pace – select an 80% solution that is 100% executable • Retain critical talent across both companies during integration for a smooth transition • Trust the integration process, while ensuring it is fit for purpose • Incorporate best practices from each company to ensure a competitive & innovative entity • Position combined entity to benefit from enhanced scale and to deliver operational synergies • "New company, new organization" positioned to sustainably deliver • Ensure early alignment on organization design to facilitate new leader participation in integration decisions and a smooth Day 1 launch • Facilitate a fair talent process respectful of the employees of both companies - great assets can only be maximized by having great people • Commit to consistent and frequent open communication as we bring our companies together • Foster a culture that honors work - life balance and rewards performance • Be honest, kind and respectful • Try not to offend, and try harder not to be offended • Remain focused on what you can control, do it safely and with integrity Execution Certainty Value Capture Organization, Talent, & Culture

2 Copyright © 2021 by Boston Consulting Group. All rights reserved. ​ Forward - Looking Statements ​ Certain statements made herein are not historical facts but are forward - looking statements for purposes of the safe harbor provi ded by Section 27A of the Securities Act of 1933, Section 21E of the Securities Exchange Act of 1934 and the Private Securities Litigation Reform Act of 1995. Forward - looking statements generally a re accompanied by words such as “may”, “should”, “would”, “plan”, “intend”, “anticipate”, “believe”, “estimate”, “predict”, “potential”, “seem”, “seek”, “continue”, “future”, “will”, “expect” , “ outlook” or other similar words, phrases or expressions. These forward - looking statements include statements regarding Whiting’s or Oasis’ industry, future events, the proposed transaction be tween Whiting and Oasis (the “proposed transaction”), the estimated or anticipated future results and benefits of the combined company following the proposed transaction, including the likeliho od and ability of the parties to successfully consummate the proposed transaction, integration of the operations of Whiting and Oasis and the outcome of such integration efforts, anticipated leve ls of free cash flow and debt, shareholder returns (including the payment of future dividends), future opportunities for the combined company, statements regarding the payment of bonuses, change of c ont rol benefits and severance benefits, and other statements that are not historical facts. These statements are based on current expectations and are not predictions of actual performance. These st atements are subject to a number of risks and uncertainties regarding Whiting’s and Oasis’ businesses and the proposed transaction, and actual results may differ materially. These risks and uncer tai nties include, but are not limited to, the possibility that stockholders of Whiting and Oasis may not approve the merger agreement; the risk that a condition to closing of the proposed transaction may not be satisfied, that either party may terminate the merger agreement or that the closing of the proposed transaction might be delayed or not occur at all; potential adverse reactions or changes to business or employee relationships, including those resulting from the announcement or completion of the transaction; the diversion of management time on transaction - related issues; the ultimate timi ng, outcome and results of integrating the operations of Whiting and Oasis; the effects of the business combination of Whiting and Oasis, including the combined company's future financial condit ion , results of operations, strategy and plans; the ability of the combined company to realize anticipated synergies in the timeframe expected or at all; changes in capital markets and the ability of t he combined company to finance operations in the manner expected; regulatory approval of the transaction; the effects of commodity prices; the risks of oil and gas activities; and the fact th at operating costs and business disruption may be greater than expected following the public announcement or consummation of the proposed transaction. ​ Additional factors that could cause results to differ materially from those described above can be found in Whiting’s Annual Rep ort on Form 10 - K for the year ended December 31, 2021 (as amended), which is on file with the Securities and Exchange Commission (the “SEC”) and available on Whiting’s website at www.whiting.com under the "Investor Relations" tab, and in other documents Whiting files with the SEC; and in Oasis’ Annual Report on Form 10 - K for the year ended December 31, 2021, which is on file with the SEC and available on Oasis’ website at www.oasispetroleum.com under the “Investors” tab, and in other documents Oasis files with the SEC. ​ All forward - looking statements speak only as of the date they are made and are based on information available at that time. Neit her Whiting nor Oasis assumes any obligation to update forward - looking statements to reflect circumstances or events that occur after the date the forward - looking statements were made or to r eflect the occurrence of unanticipated events except as required by federal securities laws. As forward - looking statements involve significant risks and uncertainties, caution should be exercised against placing undue reliance on such statements. ​ Important Additional Information Regarding the Merger Will Be Filed With the SEC ​ This communication is being made in respect of the proposed transaction involving Whiting and Oasis. The proposed transaction wi ll be submitted to stockholders of Whiting and stockholders of Oasis for their consideration and approval at a special meeting of the respective stockholders of each. In connection with the prop ose d transaction, Whiting and Oasis intend to file materials with the SEC, including a Registration Statement on Form S - 4 of Oasis (the “Registration Statement”) that will include a joint proxy statement /prospectus to be distributed to the stockholders of Whiting and the stockholders of Oasis in connection with the solicitation of proxies for the vote of the respective stockholders of each in c onn ection with the proposed transaction and other matters as described in such proxy statement. After the Registration Statement has been declared effective by the SEC, Whiting and Oasis intend to ma il a definitive joint proxy statement/prospectus (the “Definitive Proxy Statement”) and other relevant documents to their respective stockholders as of the record date established for voting on the pr oposed transaction. Investors and security holders of Whiting and Oasis are each advised to read, when available, the Registration Statement and any amendments thereto, the Definitive Proxy Stateme nt, once available, and other relevant documents to be filed by Whiting and Oasis with the SEC in connection with Whiting’s and Oasis’ solicitation of proxies for their respective special m eet ings of stockholders to be held to approve the proposed transaction, because the Registration Statement, the Definitive Proxy Statement and such other relevant documents will contain important i nfo rmation about the proposed transaction and the parties to the proposed transaction. Stockholders will also be able to obtain copies of the Registration Statement, the Definitive Proxy Sta tem ent, and other relevant documents filed by Whiting and Oasis with the SEC (when they become available), without charge, at the SEC’s website at www.sec.gov .

3 Copyright © 2021 by Boston Consulting Group. All rights reserved. ​ Copies of documents filed with the SEC by Whiting will be available free of charge at Whiting’s website at www.whiting.com under the "Investor Relations" tab or by directing a request to: Investor Relations Department at (303) 837 - 1661 or BrandonD@whiting.com. Copies of documents filed with the SEC by Oasis will be availabl e free of charge at Oasis’ website at www.oasispetroleum.com under the “Investors” tab or by directing a request to: Oasis’ Investor Relations Department at (281) 404 - 9600 or ir@oasispetroleum.com . ​ Participants in the Solicitation ​ Whiting and Oasis and their respective directors, executive officers, other members of management, and employees, under SEC r ule s, may be deemed to be participants in the solicitation of proxies of Whiting’s stockholders and Oasis’ stockholders in connection with the proposed transaction. Information regarding the directo rs, executive officers, other members of management, and employees of Whiting is included in its definitive proxy statement for its 2021 annual meeting filed with the SEC on March 29, 2021. Informat ion regarding the directors, executive officers, other members of management, and employees of Oasis is included definitive proxy statement for its 2021 annual meeting filed with the SEC on M arc h 18, 2021. Additional information regarding the persons who may, under SEC rules, be deemed participants in the solicitation of Whiting’s stockholders and Oasis’ stockholders in connection w ith the proposed transaction will be set forth in the Registration Statement, the Definitive Proxy Statement and other materials when they are filed with the SEC. ​ No Offer or Solicitation ​ This document is not intended to and does not constitute an offer to sell or the solicitation of an offer to subscribe for or bu y or an invitation to purchase or subscribe for any securities or the solicitation of any vote in any jurisdiction pursuant to the proposed transaction or otherwise, nor shall there be any sale, iss uance or transfer of securities in any jurisdiction in contravention of applicable law. Subject to certain exceptions to be approved by the relevant regulators or certain facts to be ascertained, t he public offer will not be made directly or indirectly, in or into any jurisdiction where to do so would constitute a violation of the laws of such jurisdiction, or by use of the mails or by any m ean s or instrumentality (including facsimile transmission, telephone and the internet) of interstate or foreign commerce, or any facility of a national securities exchange, of any such jurisdiction.