The Omnicom Group is one of the world's largest advertising agency holding companies in terms of revenue and one of the "Big 6" advertising holding companies; the other companies being WPP Group plc, Interpublic, Publicis, Dentsu and Havas. Its major holdings include the advertising agency networks BBDO, TBWA Worldwide, and DDB Worldwide. Its portfolio of companies also includes public relations, lobbying, marketing services, interactive design, CRM specialists and media buying services.
Omnicom's specialty marketing and public relations agency holdings are managed by its Diversified Agency Services division. Among Omnicom's PR agency holdings are Brodeur Worldwide, Fleishman-Hillard, Ketchum Inc., Luntz-Maslansky Strategic Research and Porter Novelli. In 2005, Omnicom acquired its first search engine marketing firm, Chicago-based Resolution Media, and its first brand licensing consultancy, The Beanstalk Group.
Omnicom was formed in 1986 from the merger between advertising agency networks DDB Needham and BBDO. Sometimes referred to as the "Big-Bang" merger, it was spearheaded by BBDO Worldwide CEO Allen Rosenshine, who was responding to competitive threats from other large advertising agency comglomerates. In 1993 Omnicom acquired TBWA Worldwide, a major advertising agency network.