Ovation Marketing Inc.
- An employee-owned advertising agency in La Crosse, Wisconsin.
- http://www.ovationadvertising.com/

Ontrac Group
- International agency offering complete marketing and design solutions across all sectors. Offices in Bristol, Cardiff and Newtown (UK) and New York. [English and Welsh versions.]
- http://www.ontrac-group.com/

Oberoi Multimedia Ltd.
- A Mumbai-based agency with the prime objective of brand development and management through advertising, public relation, event management graphics design, media and strategic planning.
- http://www.oberoimultimedia.com/

Osborn and Barr Communications
- St. Louis and Kansas City agency focuses on market segmentation, brand management, product branding and sales promotion.
- http://www.osborn-barr.com/

O'Grady Meyers
- Overview of agency provided.
- http://www.ogradymeyers.com/

Osborn & DeLong
- Osborn & DeLong is a full-service design agency located in Bloomington, Illinois, specializing in print design as well as new media.
- http://www.osborndelong.com

The ORCA Partnership
- Full service St. Louis, MO based advertising and marketing agency.
- http://www.orcastl.com/

Ogilvy & Mather
- International advertising, marketing and public relations agency founded in 1948.
- http://www.ogilvy.com/

OIA Marketing Communications
- Full service advertising agency specializing in business-to-business marketing and technical communications.
- http://www.oia-inc.com/

Oblik Creation Marketing, Inc
- A 20-year old Montreal Advertising Agency which relies on freelancers rather than staff to create advertising.
- http://www.oblik.com/

O'Brien et al. Advertising
- Advertising, it's a rhythm. Wanna dance?
- http://www.obrienetal.com/