| OpenArena team | Developer | |
| OpenGAMES studios | Developer | |
| OpenRA developers | Developer | |
| OpenSesame Inc. | Developer | |
| OpenSorceryGames | Developer | |
| OpenTTD Team | DeveloperPublisher | |
| Openfun | Developer | |
| Opensystem | Developer | |
| Opera House | Developer | |
| Opera House Corporation | Developer | |
| Opera House inc | Developer | |
| Opera Soft | DeveloperPublisher | |
| Opera Soft S.A. | Publisher | |
| Opera Sports | Developer | |
| OperaHouse | DeveloperPublisher | |
| OperaHouse Corporation | Developer | |
| Ophaneom | Developer | |
| OpheliaEnigma | Developer | |
| Opossum In A Jar | Developer | |
| Opqam | DeveloperPublisher | |
| Ops Game Studio | Developer | |
| Opsage | Developer | |
| Opsie_Games | Developer | |
| Optical Hex Games | Developer | |
| OpticalOverride | Developer | |
| Optigear Studios | Developer | |
| Optik Software | DeveloperPublisher | |
| Optillusion | Developer | |
| OptimalScrub | Developer | |
| Optimisto | Developer | |
| Optimum Resource | Publisher | |
| Optimum Resource, Inc. | DeveloperPublisher | |
| Optimus Software | Developer | |
| Optimus Software (Jason Falcus, Adrian Ludley, David Whittaker) | Developer | |
| Optimus Software (Jason Falcus, Adrian Ludley, Lyndon Sharp) | Developer | |
| Optimus Software (Jason Falcus, Adrian Ludley, Paul Hiley) | Developer | |
| Optimus Software (Neil Hill, Adrian Ludley, Andrew Severn) | Developer | |
| Option 2 Games | Developer | |
| OptionalGames | Developer | |
| Optyk | Publisher | |
| Optyx | Publisher | |
| Opus | Developer | |
| Opus Company | Developer | |
| Opus Corp | Developer | |
| Opus Corporation | Developer | |
| Orace | Developer | |
| Oracion | DeveloperPublisher | |
| Oracle & Bone | Developer | |
| Oracle Animosity | Developer | |
| Oracle Software | Publisher | |