APC Anadarko Petroleum Corporation

Filed by Anadarko Petroleum Corporation

pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934

Subject Company: Anadarko Petroleum Corporation

Commission File No.: 001-08968

On May 13, 2019, Anadarko Petroleum Corporation (“Anadarko”) posted the following letter from the President and Chief Executive Officer of Occidental Petroleum Corporation (“Occidental”) to an internal website available to Anadarko’s employees.

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Dear Anadarko team,

I’m excited to have the opportunity to reach out to all of you to introduce myself and share some important news. The combination of Occidental and Anadarko will create a $100 billion global energy leader with the scale, assets and efficiencies to lead our industry into the future and benefit stakeholders of both companies.

We will have an even more competitive business through a powerful mix of world-class assets, and to put this in perspective:

We expect to deliver organic production growth of 5 percent while accelerating our shareholder returns-focused strategy. And we will remain committed to a future where we extend carbon dioxide utilization and sequestration globally, to truly lead energy into a low-carbon future.

You may be wondering what happens next. We are at the beginning of a longer process to combine our two companies. It’s important to remember that until the transaction is complete, Occidental and Anadarko will remain separate companies.

In the coming weeks, I will be visiting your operations and offices to meet with employees. I have asked Glenn Vangolen, Occidental’s Senior Vice President of Business Support, to create an integration team with members from both Occidental and Anadarko. He and I both look forward to meeting and working with you to ensure the integration is efficient. Change creates uncertainty and stress, which we hope to minimize as much as possible as we go through the planning prior to close and the implementation after close.

Our hope is to create a company that takes the best practices of Occidental and Anadarko applied to great assets and driven by incredible employees from each company. We expect that our culture will also be a blend of the best traits from each, which will ultimately result in a high-performing company with engaged, empowered and fulfilled employees.

I am enthusiastic about the opportunities that lie ahead for our combined company and the talented people who will ensure its success for the next 100 years.

Sincerely,

Vicki Hollub

President and Chief Executive Officer

Important Information For Investors And Stockholders

This communication does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended. In connection with the potential transaction, Occidental expects to file a registration statement on Form S-4 with the Securities and Exchange Commission (“SEC”) containing a preliminary prospectus of Occidental that also constitutes a preliminary proxy statement of Anadarko. After the registration statement is declared effective Anadarko will mail a definitive proxy statement/prospectus to stockholders of Anadarko. This communication is not a substitute for the proxy statement/prospectus or registration statement or for any other document that Occidental or Anadarko may file with the SEC and send to Anadarko’s stockholders in connection with the potential transaction. INVESTORS AND SECURITY HOLDERS OF OCCIDENTAL AND ANADARKO ARE URGED TO READ THE PROXY STATEMENT/PROSPECTUS AND OTHER DOCUMENTS FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION. Investors and security holders will be able to obtain free copies of the proxy statement/prospectus (when available) and other documents filed with the SEC by Occidental or Anadarko through the website maintained by the SEC at http://www.sec.gov. Copies of the documents filed with the SEC by Occidental will be available free of charge on Occidental’s website at http://www.oxy.com/investors and copies of the documents filed with the SEC by Anadarko will be available free of charge on Anadarko’s website at http://investors.anadarko.com.

Occidental and Anadarko and certain of their respective directors, certain of their respective executive officers and other members of management and employees may be considered participants in the solicitation of proxies with respect to the potential transaction under the rules of the SEC. Information about the directors and executive officers of Occidental is set forth in its Annual Report on Form 10-K for the year ended December 31, 2018, which was filed with the

SEC on February 21, 2019, and its proxy statement for its 2019 annual meeting of stockholders, which was filed with the SEC on March 28, 2019. Information about the directors and executive officers of Anadarko is set forth in its Annual Report on Form 10-K for the year ended December 31, 2018, which was filed with the SEC on February 14, 2019, and its proxy statement for its 2019 annual meeting of stockholders, which was filed with the SEC on March 29, 2019. These documents can be obtained free of charge from the sources indicated above. Additional information regarding the interests of such participants in the solicitation of proxies in respect of the potential transaction will be included in the registration statement and proxy statement/prospectus and other relevant materials to be filed with the SEC when they become available.

Cautionary Statement Regarding Forward-Looking Information

This communication contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. Anadarko believes that its expectations are based on reasonable assumptions. No assurance, however, can be given that such expectations will prove to have been correct. A number of factors could cause actual results to differ materially from the projections, anticipated results or other expectations expressed in this news release, including regarding the proposed transaction. These include the timing, receipt and terms and conditions of any required governmental and regulatory approvals of the proposed transaction that could reduce anticipated benefits or cause the parties to abandon the proposed transaction, the occurrence of any event, change or other circumstances that could give rise to the termination of the merger agreement, the possibility that stockholders of Anadarko may not adopt the merger agreement, the risk that the parties may not be able to satisfy the conditions to the proposed transaction in a timely manner or at all, risks related to disruption of management time from ongoing business operations due to the proposed transaction, the risk that any announcements relating to the proposed transaction could have adverse effects on the market price of Anadarko’s common stock or Occidental’s common stock, the risk of any unexpected costs or expenses resulting from the proposed transaction, the risk of any litigation relating to the proposed transaction, the risk that the proposed transaction and its announcement could have an adverse effect on the ability of Anadarko or Occidental to retain customers and retain and hire key personnel and maintain relationships with their suppliers, customers and other business relationships and on their operating results and businesses generally, the risk the pending proposed transaction could distract management of both entities and they will incur substantial costs, the risk that problems may arise in successfully integrating the businesses of the companies, which may result in the combined company not operating as effectively and efficiently as expected, the risk that the combined company may be unable to achieve synergies or other anticipated benefits of the proposed transaction or it may take longer than expected to achieve those synergies or benefits and other important factors that could cause actual results to differ materially from those projected. All such factors are difficult to predict and are beyond Anadarko’s control. Additional factors that could cause results to differ materially from those described above can be found in Anadarko’s most recent Annual Report on Form 10-K, as it may be updated from time to time by quarterly reports on Form 10-Q and current reports on Form 8-K all of which are available on Anadarko’s website at http://investors.anadarko.com/sec-filings and on the SEC’s website at http://www.sec.gov, and in Occidental’s most recent Annual Report on Form 10-K, as it may be updated from time to time by quarterly reports on Form 10-Q and current reports on Form 8-K all of which are available on Occidental’s website at http://www.oxy.com/investors and on the SEC’s website at http://www.sec.gov.