- Ominous Development - Developer of Onyx Star.
- OmletteSoft, Home Of Omlette - Lord Omlette's personal homepage.
- On-Line Entertainment - Publishers of Automayhem, Art of Kings, and Iron Wolves.
- Ongame e-solutions - Developer of online poker games, using Java technology.
- Oquirrh Productions - Developers of hunting, shooting and conservation games.
- OTS Software - Developer of freeware, shareware, and inexpensive retail games, including card games, puzzles, first-person shooters.
- Outsource Media - Offers computer games voice production in the UK, US. Services include casting, recording, post production, game developer services, and support.