OMNICOM GROUP INC

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

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PROXY STATEMENT

Annual Meeting of Shareholders

May 7, 2024, at 10:00 a.m.

Eastern Daylight Time

Flywheel Digital

1801 Porter Street, Suite 300

Baltimore, MD 21230

A Letter from Omnicom’s Lead Independent Director

To My Fellow Shareholders:

It is a great honor to be writing my first letter to you as Omnicom’s Lead Independent Director, a role I assumed on January 1, 2024. I want to thank my predecessor and colleague on the Board, Len Coleman, for his tremendous leadership in the position over the past eight years, and for his continued support and guidance during our transition of responsibilities. As I reflect on Omnicom’s accomplishments throughout 2023, I am incredibly proud of our successful execution of strategic business priorities, our commitment to growing and supporting Omnicom’s best-in-class talent, and our Board’s continued growth as a diverse, skilled, and engaged group of directors.

In my new independent leadership position, I am committed to ongoing collaboration with my fellow Board members and Omnicom’s management team to realize our core objective of generating long-term value for our shareholders. Direct shareholder engagement and the feedback we receive from those engagements remain critical components of the process our Board and management undertake to achieve that objective, and I have enjoyed engaging and look forward to continuing to engage in constructive dialogue with shareholders in my new role. I also look forward to continuing our long-standing practice of actively obtaining feedback on key focus areas for the Board and management, including those discussed below, in the coming year.

Execution on Strategic Initiatives. Our leadership team delivered strong operational and financial performance in 2023, while navigating a complex economic environment, by continuing to expand our capabilities and drive areas of growth through investment. For example, on January 2, 2024, we closed the acquisition of Flywheel Digital, which significantly broadens our capabilities in e-commerce, retail media and high-growth digital areas, as we continue to deliver superior results for our clients and outpace our competition. In addition to executing key strategic acquisitions during the year, we established first-mover technology partnerships with leading generative artificial intelligence (AI) companies, such as Microsoft, Adobe, AWS, and Getty Images. We are also thoughtfully integrating AI into our market-leading technology platform, Omni, and leveraging AI to enhance user experiences as we build, expand, and improve our client offerings, all while carefully considering and evaluating the associated risks.

Ongoing Board Refreshment. We have evolved our Board in recent years through a deliberate and ongoing refreshment process, which incorporates fresh perspectives and prioritizes a mix of skills, experience, and diversity to provide effective oversight of the Company’s strategy. Seven of our 11 nominees this year are women, four are Black, and two are Hispanic/Latinx. Additionally, all independent Board leadership positions are currently held by gender or ethnically diverse directors. On January 1, 2024, Cassandra Santos joined our Board, bringing significant expertise in technology, cybersecurity, digital transformation, and AI to the Board and strengthening our collective skillset.

Promoting a Diverse, Equitable, and Inclusive Workplace. Talent remains a core differentiator for Omnicom, and we seek to cultivate a diverse workforce at all levels that drives creativity and innovation for our clients. We consistently ground our work in data-driven results, and in 2023 we saw meaningful progress against our five workforce Key Performance Indicators (KPIs). We also raised the bar for ourselves and for our industry by introducing two new goals to advance DE&I, aimed at expanding BIPOC representation across our workforce and increasing the number of women in leadership positions. In 2024, we are launching our OPEN 3.0 Initiative, which will further guide and strengthen our efforts to operationalize DE&I across our business. As part of this broader vision, Omnicom launched our first-ever Global Steering Committee to develop globally inclusive and culturally relevant strategies for our clients and communities. Additional information can be found in our second annual standalone DE&I report.

Advancing Environmental Sustainability and Responsible Practices. We continue to make significant progress against our environmental sustainability priorities, which are overseen by the Governance Committee. We recently set ambitious goals to reduce our Scope 1, 2, and 3 global emissions by 46.2% by 2030, which is aligned with our corporate strategy to consolidate offices and optimize the use and efficiency of our spaces. Our near-term emissions-reduction target was validated by the Science Based Targets initiative (SBTi) in early 2023. In addition, through our work as a founding member of Global Ad Net Zero, we play a leading role in efforts to reduce carbon emissions from advertising operations and find innovative ways to strengthen our sustainability initiatives. Our environmental sustainability initiatives and progress against key goals are reflected in our most recent Corporate Responsibility report.

On behalf of the full Board of Directors, thank you for your investment in Omnicom and consideration of the important matters set forth in our Proxy Statement. It was a successful 2023 for our Company, and I look forward to working with the Board and management this year in my new role, as Omnicom continues to provide world-class services to our clients and deliver value for our shareholders, people, and communities.

Mary C. Choksi

Lead Independent Director

2 2024 Proxy Statement

NOTICE OF 2024 ANNUAL MEETING OF

SHAREHOLDERS

This summary highlights selected information about the items to be voted on at the 2024 Annual Meeting of Shareholders (or “2024 Annual Meeting”). This summary does not contain all of the information that you should consider in deciding how to vote. You should read the entire Proxy Statement carefully before voting.

Meeting Agenda and Voting Recommendations

ITEM 1: Election of Directors

The Board recommends a vote FOR each of the director nominees. ■ We have conducted a comprehensive evaluation of director skill sets to enable each director’s unique qualifications and attributes to collectively support the oversight of Omnicom’s management. ■ Each of our directors is elected annually by a majority of votes cast. ■ 10 of Omnicom’s 11 director nominees are independent, and each of the Audit, Compensation, Governance and Finance Committees is comprised solely of independent directors. ■ Diversity is a core value across our organization. A majority of our director nominees are women, four are Black and two are Hispanic/Latinx. The Audit, Compensation and Finance Committees are all Chaired by directors who are women, and the Chair of the Governance Committee is Black.

See page 13for furtherinformation

DIRECTOR NOMINEES

4 2024 Proxy Statement

Board Nominees Snapshot

Experience and Skills

Our director nominees are accomplished leaders who bring a mix of experiences and skills to the Board.

Our Board has identified skill categories fundamental to its ability to effectively oversee Omnicom’s strategy and management, and undertakes a comprehensive evaluation so that these skills are well represented on the Board.

One new director joined the Board on January 1, 2024 as part of our continued refreshment process to bring fresh perspectives to the Board. Our new director also brings relevant technology, cybersecurity, digital transformation and AI expertise, which strengthens our Board’s ability to effectively oversee management’s execution against business strategy.

See pages 14 and 15 for further information.

GOVERNANCE HIGHLIGHTS

The Board has adopted, and periodically reviews, policies and procedures to guide it in its oversight responsibilities. These policies and procedures provide a framework for the proper operation of our Company and align with shareholders’ interests.

SHAREHOLDER ENGAGEMENT

Ongoing shareholder engagement is a priority for our Board and management team. In the fall of 2023, we reached out to shareholders holding 70% of our outstanding shares with an invitation to engage. We spoke to every shareholder that accepted our invitation, which represented an aggregate of 22% of our outstanding shares. As in prior years, our Lead Independent Director was an active participant in select shareholder meetings. Broad topics discussed included:

■Company strategy and performance

■Board composition, refreshment and leadership

■Lead Independent Director succession process

■Management succession planning

■Executive compensation program

■Diversity, Equity and Inclusion (“DE&I”) efforts across the organization

■Environmental sustainability initiatives

■Human capital management initiatives

■Governance practices

6 2024 Proxy Statement

We have made a number of changes in recent years in response to the feedback we have received from our ongoing shareholder engagement efforts, which management regularly reviews with the Board, including:

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We appreciate the insights and perspectives of our shareholders, which were discussed among the full Board.

ITEM 2: Advisory Resolution to Approve Executive Compensation

The Board recommends a vote FOR this voting item. ■ We closely tie pay to current and long-term Company performance; ■ We maintain a high degree of variable “at-risk” compensation; ■ We establish challenging quantitative performance metrics that align with our business strategy, which determine 80% of our 2023 Annual Incentive Award; ■ We determine 20% of our 2023 Annual Incentive Award by assessing certain qualitative performance metrics, including DE&I, environmental sustainability, human capital management and employee training initiatives, and corporate responsibility and integrity; and ■ We sustain competitive compensation levels.

See page 42for furtherinformation

EXECUTIVE COMPENSATION HIGHLIGHTS

2023 Performance Overview and Highlights

Omnicom finished 2023 on a strong note. Our full year revenue was $14,692.2 million, with organic growth of 4.1%; operating income was $2,104.7 million and adjusted operating income was $2,231.9 million; operating margin was 14.3% and adjusted operating margin was 15.2%; and diluted EPS was $6.91 and adjusted diluted EPS was $7.41. We generated approximately $1.4 billion in net cash provided by operating activities, an increase of 53.4% compared to the full year 2022, and approximately $1.9 billion in free cash flow, an increase of 6.5% compared to the full year 2022. We returned $1.1 billion to shareholders through dividends and share repurchases and ended the year with $4.4 billion in cash and cash equivalents. See Annex A for the definitions of adjusted operating income, adjusted operating margin, adjusted diluted EPS and free cash flow, which are non-GAAP measures, and a reconciliation of each non-GAAP measure to the most directly comparable GAAP measure. Our return on equity was 40.5% in 2023, while our return on invested capital was 25.8%. As we move into 2024, Omnicom’s liquidity and balance sheet remain strong and continue to support our primary uses of cash — dividends, acquisitions and share repurchases. Throughout the year, we continued to invest in areas that will shape the future of our industry through acquisitions and internal investments and partnerships.

In determining the Annual Incentive Award for performance in 2023, the Compensation Committee considered Company (i.e., internal) financial performance (weighted at 40%), our performance as compared with the performance of our peer group (weighted at 40%) and certain qualitative metrics (weighted at 20%) in order to increase alignment between Company performance and pay outcome. In assessing qualitative metrics, the Compensation Committee’s intent is to promote accountability for progress on our DE&I initiatives, environmental sustainability, human capital management and employee training initiatives, and corporate responsibility and integrity.

The chart below outlines the metrics used in determining our Annual Incentive Award for 2023, which were the same as those metrics to determine the 2022 Annual Incentive Award, with the exception of the removal of the response to COVID-19 challenges from the qualitative performance metric. The program is discussed in greater detail below in the section entitled “Calculation of Annual Incentive Award” on page 57.

10 2024 Proxy Statement

Compensation Best Practices

Emphasis on performance-based compensation Executive and director stock ownership guidelines (6x base salary for Chairman and CEO and for President and COO; 3x base salary for CFO; 5x annual cash retainer for directors) Policy adopting equity grant best practices New compensation recovery (i.e., clawback) policy in compliance with U.S. Securities and Exchange Commission (“SEC”) and New York Stock Exchange (“NYSE”) rules Policy prohibiting hedging of Company equity securities Policy prohibiting pledging and margin transactions

ITEM 3: Ratify the appointment of KPMG LLP as our independentauditors for the fiscal year ending December 31, 2024