| OniWasARacecarDriver | Developer | |
| Onigiri Seisakusho | Developer | |
| OniiOniiHopper | Developer | |
| Onildo Aguiar | Developer | |
| Onion Egg | Developer | |
| Onion Games | DeveloperPublisher | |
| Onion Samson | Developer | |
| Onion Software | Developer | |
| Onion Soup Interactive | DeveloperPublisher | |
| OnionBlaze | Developer | |
| OnionCorp | Developer | |
| Onionman77 | Developer | |
| OniriCorpe | Developer | |
| Oniria World | Developer | |
| Onirico Studios | Developer | |
| OnlineChungus | Developer | |
| OnlyGames | Developer | |
| OnlyPurpleRain | Developer | |
| OnlyShellder | Developer | |
| OnlySlightly | Developer | |
| Onlyjoy`s production | Developer | |
| Onslaught Supply Games | Developer | |
| Onstorm | Developer | |
| Onteca | DeveloperPublisher | |
| Onvisoc | Developer | |
| Onybr | Developer | |
| Onygox | Developer | |
| Onyric Productions | Developer | |
| Onyx Lute | Developer | |
| OnzaEstudio | Developer | |
| OofyDoodle | Developer | |
| Oogaphobia | Developer | |
| Ookami Kasumi | Developer | |
| Ooniversal | Developer | |
| Ooops24 | Developer | |
| Oopa Chaloopa | Developer | |
| Ooparts | Developer | |
| Oops | DeveloperPublisher | |
| Oops! | Developer | |
| Opa-Opa | Developer | |
| Opal Starlight | Developer | |
| Opalir | Developer | |
| Opaque Media | Developer | |
| Open Book | DeveloperPublisher | |
| Open Corporation | Developer | |
| Open Door Games | Developer | |
| Open House Games | Developer | |
| Open Lab Games | Developer | |
| Open Loop Games | Developer | |
| Open Sesame | Developer | |
| 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 | |