UNITED STATES SECURITIES AND EXCHANGE COMMISSION FORM 10-K OMNICOM GROUP INC. FOR THE FISCAL YEAR ENDED DECEMBER 31, 2020 Washington, D.C. 20549 ☑ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number: 1-10551 New York (State or other jurisdiction of incorporation or organization) 280 Park Avenue, New York, NY (Address of principal executive offices) 13-1514814 (Exact name of registrant as specified in its charter) (I.R.S. Employer Identification No.) 10017 (Zip Code) Registrant’s telephone number, including area code: (212) 415-3600 Securities Registered Pursuant to Section 12(b) of the Act: Securities Registered Pursuant to Section 12(g) of the Act: None Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes þ No o Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act. Yes o No þ Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports) and (2) has been subject to such filing requirements for the past 90 days. Yes þ No o Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant was required to submit such files). Yes þ No o Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act. Large accelerated filer ☑ Smaller reporting company ☐ Accelerated filer ☐ Non-accelerated filer ☐ Emerging growth company ☐ 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 13(a) of the Exchange Act. o Indicate by check mark whether the registrant has filed a report on and attestation to its management’s assessment of the effectiveness of its internal control over financial reporting under Section 404(b) of the Sarbanes-Oxley Act (15 U.S.C. 7262(b)) by the registered public accounting firm that prepared or issued its audit report. ☑ Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes ☐ No þ The aggregate market value of the voting and non-voting common stock held by non-affiliates as of June 30, 2020 was $11,583,494,559. As of February 11, 2021, there were 215,014,390 shares of Omnicom Group Inc. Common Stock outstanding. Portions of the Omnicom Group Inc. Definitive Proxy Statement for the Annual Meeting of Shareholders scheduled to be held on May 4, 2021 are incorporated by reference into Part III of this report to the extent described herein. Title of each class Trading Symbols Name of each exchange on which registered Common Stock, $0.15 Par Value OMC New York Stock Exchange 0.800% Senior Notes due 2027 OMC/27 New York Stock Exchange 1.400% Senior Notes due 2031 OMC/31 New York Stock Exchange