Omnicom Group, Inc. is an American global marketing and corporate communications holding company, headquartered in New York City.

Omnicom's branded networks and specialty firms provide services in four disciplines: advertising, customer relationship management (CRM), public relations and specialty services. The services included in these disciplines are media planning and buying, digital and interactive marketing, sports and events marketing, field marketing and brand consultancy. Omnicom Group was ranked as one of the four largest advertising agencies in the world by The New York Times in 2002. In 2014, Omnicom was considered the second largest advertising holding company by The Wall Street Journal. The company employs more than 74,000 employees in about 100 countries worldwide.

In 1986, Allen Rosenshine, Keith Reinhard and John Bernbach co-created Omnicom in a three-way merger of BBDO Worldwide, Doyle Dane Bernbach and Needham Harper Worldwide. Dubbed the "Big Bang", the merger aligned each agency on the founding principle of creativity. In 1989, Rosenshine stepped down as chairman of Omnicom to return to his role running BBDO Worldwide and Bruce Crawford was put in place as the chairman of Omnicom. In 1997, John Wren, the number two executive at Omnicom, moved up to the CEO role, while Crawford remained chairman. Under Wren, the company has grown into the second largest agency holding company and had revenue of over $15 billion in 2014. According to the company's website the agency now services over 5,000 brands.

Omnicom operates as a holding company and is composed of five major agency networks that oversee 1500+ agencies as parent companies. The networks are BBDO Worldwide, Diversified Agency Services (DAS), DDB Worldwide, Omnicom Media Group (OMG) and TBWA Worldwide.

BBDO Worldwide is a creative agency within the Omnicom network and was one of the companies that took part in the initial merger that formed Omnicom. In addition to being recognized as one of the most creative networks in the world, several of the BBDO agencies were ranked in the top 50 awarded individual agencies in the world by The Gunn Report. Founded in 2000, Proximity Worldwide is BBDO's digitally focused marketing arm. In January 2016, BBDO Worldwide acquired a majority stake in Wednesday Agency Group. The agency also owns Sellbytel.

The DAS Group of Companies is a group of marketing services that includes over 200 companies in the following marketing disciplines: public relations, CRM, healthcare, events, promotional marketing, vehicle testing, branding and research. Some of the businesses included in the DAS network are Alcone, Omnicom Health Group, Automotive Marketing Consultants Inc., Critical Mass, FleishmanHillard, Goodby, Silverstein & Partners, The Marketing Arm, Porter Novelli, maslansky + partners, Doremus, and C Space.

DDB Worldwide is one of the three advertising networks, it offers brand building, campaign management and creative advertising development. Some of the businesses included in the DDB Worldwide unit are Tribal Worldwide, Adam & Eve DDB, gutenberg global, Roberts + Langer, Spike DDB, TracyLocke, Interbrand, and Uproar!@DDB. In November 2015, DDB Worldwide acquired Grupo ABC, the largest advertising group in Brazil.

Omnicom Media Group is the media division of the Omnicom Group Inc. Companies operating in their network include media networks OMD Worldwide, PHD Worldwide, Hearts & Science, data and analytics company, Annalect, and performance media firm, Resolution Media.

TBWA is a global agency network with a philosophy of "disruption" is normally billed as an alternative agency. It was purchased in 1993 and then merged with the Chiat Day firm in 1995. In the United States some of the company's offices still retain the TBWA Chiat Day name. Companies operating under the TBWA network include, Digital Arts Network (DAN), The Integer Group, GSD&M, Zimmerman Advertising, Martin Williams, Designory, NEBOKO , and other TBWA branded regional agencies.