Omnicom Omnicom Group is one of the four largest advertising groups in the world (the other three are,,) and is a publicly traded company (NYSE: OMC). Omnicom Group was founded in 1986, headquartered in New York, U.S., operating businesses including branding, marketing, public relations, market research, etc., owned by Everyman Advertising (DBB), Horton Media and other companies, announced a merger with the French Publicis Group in 2013, and later announced in 2014 to cancel the merger.