O
- O&G Industries
- O, The Oprah Magazine
- O-Town (musician)
- O.P.I (designer)
- Oak Ridge National Laboratory
- Oakland Athletics
- Oakland Golden Grizzlies (basketball)
- Oakland Museum
- Oakland Raiders
- Oakland University
- Oaklawn Park
- Oakley, Inc.
- Oasis (Band)
- Oasis Infotech
- Oasis Media Matrix
- Oasis Securities
- Oatfield (confectionery)
- Oberlin College
- Oberoi Plastics
- Oberoi Realty
- Obi Best
- Obie Trice (musician)
- Obili Infrastructure Private Limited
- Obsidian Entertainment, Inc
- OC Music Magazine
- OC Weekly
- Occidental Brothers Dance Band International
- Occidental College
- Occidental Petroleum Corporation
- Occupational Safety and Health Administration
- Occupy Wall Street
- Ocean Agro (India)
- Ocean City Barons
- Ocean Infrastructure
- Oceana
- Oceana Software Solutions
- Oceaneering International, Inc.
- Oceania Cruises
- Oceanic Magnetics
- OCL India
- Ocote Soul Sounds
- Octane TV
- Octanorm India
- Octant Interactive Technologies
- Octav Investments
- ODC
- Odd Future
- Odd Molly (designer)
- Oddworld Inhabitants
- Odebrecht S.A.
- Odfjell SE
- Odyssey Financial Services
- Odyssey Technologies
- Odyssey Video Communications
- OECD
- Oenobiol
- Of Mice & Men
- Of Montreal
- Ofcom
- Ofer Brothers Group
- Off-Track Betting Corp
- Office Depot, Inc.
- Office for National Statistics
- Office of Congressional Ethics
- Office of Federal Housing Enterprise Oversight
- Office of Management and Budget
- Office of National Drug Control Policy
- Office of Public Works (Ireland)
- Office of Research Integrity
- Office of the HM Paymaster General
- OfficeMax
- OGC Nice
- Ogden Outlaws
- OGE Energy
- Ogilvy & Mather
- Oglethorpe University
- OGX
- Ohio Bobcats (basketball)
- Ohio Bobcats (football)
- Ohio Casualty
- Ohio Elite Soccer Academy
- Ohio Northern University
- Ohio State Buckeyes (basketball)
- Ohio State Buckeyes (football)
- Ohio State University
- Ohio Theater
- Ohio University
- Ohio Wesleyan University
- Ohne Titel (designer)
- Oi (telecommunications)