O'Reilly & Assoc. (US)
O2 (UK/Global)
OAA - Outdoor Advertising Association (UK)
OAAA - Outdoor Advertising Association of America (US)
OAAC - Outdoor Advertising Association of Canada (Canada)
OAG Worldwide (US/Global)
Obie Media (US)
OBOP (Poland)
Occidental Chemical Corporation (US)
Ocean Spray (US)
Octagon (US)
Oddbins (UK)
Oddcast (US)
OddsOnSport (UK)
Odeon (UK)
OECD (France)
Office Depot (US)
Office of Population Research (US)
Office of the e-Envoy (UK)
OfficeMax (US)
OFTEL Consumer Research (UK)
Ogilvy (US)
Oglasnik (Croatia)
Ohal (UK)
OJD - Oficina de Justificacion de la Difusion (Spain)
Olay (US/Global)
Old Maps (UK)
Old Maps (UK)
Old Mutual (UK)
Old Spice (US)
Olean (US)
Olive Garden (US)
Olivetti (Italy)
Olympic Airways (Greece)
Olympus (US)
OMA - Open Mobile Alliance (UK)
OMD (US/Global)
Omega (Switzerland)
OMI - Outdoor Media International (US)
Omnicom (US)
ONcomment (UK)
oneFN (US)
OneSource Information Services (US)
Oneswoop (UK)
OneTravel (US)
OneWorld (UK)
Online Advertising Discussion List (US)
Online banking
Online Banking Report (US)
Online marketing
Online Media Guide (UK)
Online Vacation Mall (US)
Online Weather (UK/Ireland)
Online-garden (UK)
Onpack Media (UK)
Onsale Online Auctions (US)
OnTheFly Travel (US)
OoHSolutions (US)
OPA - Online Publishers Association (US)
Opel (US/Global)
OpenTV (US)
Openwave (US)
Opinionpanel (UK)
OPMA - Overseas Press & Media Association (UK)
Opodo (UK)
Opt-In News (US)
OPTA Soccer (UK)
Optimad (UK)
Optimedia (Global)
Optimedia (UK)
Optimedia (US)
Options Direct (UK)
Options Strategy (Australia)
OptiView (US)
Opus (UK)
Oracle (US)
Orajel (US)
Orange (UK)
Orchard FM (UK)
Ore-Ida (US)
ORF (Austria)
Organics Direct (UK)
Origin Search (UK)
Orkin Exterminating Company, Inc. (US)
Ornico Media (South Africa)
Orvis (US)
Osterreichischer Automobile, Motorcycle and Touring Club (Austria)
Otherland Toys (UK)
Otsuka (Japan)
Ottakars (UK)
Out There News (UK)
Outdoor and Ambient Media Directory (UK)
Outdoor Connection (UK)
Overture (US)
Overture - Search Term Suggestion Tool (US)
Ovie Entertainment (US)
Ovum (UK)
OWR - Osterreichisher Werberat (Austria)
Oxford Shoe Company (UK)
Oxford University Press (UK)
Oxigen (UK)
Oxygen (US)
OzEmail (Australia)
