| 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 | |
| Orange | DeveloperPublisher | |
| Orange Flag | Developer | |
| Orange Games | Developer | |
| Orange House | DeveloperPublisher | |
| Orange Juice Dog | Publisher | |
| Orange One | Developer | |
| Orange Pixel | DeveloperPublisher | |
| Orange Pylon | Developer | |
| OrangeeZ | Developer | |
| Orangepixel | DeveloperPublisher | |
| Orator | Developer | |
| Oraylien | Developer | |
| Orb | Developer | |
| Orbit Studio | Developer | |
| Orbital Media | Developer | |
| Orbital Media, Inc. | DeveloperPublisher | |
| Orbital Speed Studio | DeveloperPublisher | |
| OrbitalNine | Developer | |
| OrbitalShark | Developer | |
| OrbitalWorks | Developer | |
| Orblus | Developer | |
| Orc Wizard | Developer | |
| Orca | Publisher | |
| Orcalancelulu | Developer | |
| Orchid Games | Developer | |
| Orcolom | Developer | |
| Ordilogic Systems | Developer | |
| Oreganik | Developer | |
| Oregin | Developer | |
| Oren Erlich | Developer | |
| Orenda | Publisher | |
| Orenge | Developer | |
| Orenge Crest | Developer | |
| Orenji_games | Developer | |
| Orenki | Developer | |
| Oreolek | Developer | |
| Oretal | Developer | |
| Orfeas Game Studio | Developer | |
| OrfeasWorldGames Studio | Developer | |
| OrgAction Games | Developer | |
| Organa | Developer | |
| Ori Brusilovsky | Developer | |
| Oriental | Publisher | |
| Oriental Soft | Publisher | |
| Origame Digital | Developer | |
| Origin | Publisher | |
| Origin Systems | DeveloperPublisher | |
| Original Fire Games | Developer | |
| Original Gamer Stevie Strow | Developer | |
| OriginalFei | Developer | |
| Origo Games | Publisher | |
| Orihaus | Developer | |
| Orikami | Developer | |
| Oriol | Developer | |
| Orion | Developer | |
| Orion Game Studio | Developer | |
| Orion Games | Developer | |
| Orion Interactive | DeveloperPublisher | |
| Orion Software | DeveloperPublisher | |
| OrionDark7 | Developer | |
| OrionGD | Developer | |
| Orion_ | Developer | |
| OrkybitGames | Developer | |
| Orla | Developer | |
| Orlando Araujo | Developer | |
| OrlandoIsSoCool | Developer | |
| Ornith | Developer | |
| Ornithopter Games | Developer | |
| Oro | Developer | |
| Orpheck Inc. | Developer | |
| Orpheus | DeveloperPublisher | |
| Orpheus (Richard Wilkins, Geoff M. Phillips) | Developer | |
| Orpheus Software | Publisher | |
| Orrin | Developer | |
| Orsailius | Developer | |
| Orthogonal Games | Developer | |
| Orube Game Studio | DeveloperPublisher | |
| OsXurelazos | Developer | |
| Osarion | Developer | |
| Osaris Games | Developer | |
| Osariusz | Developer | |
| Oscar | Developer | |
| Oscar Arcane | Developer | |
| Oscar Brittain | Developer | |
| Oscar Celestini SHOP | Developer | |
| Oscar Cerro | Developer | |
| Oscar Chu | Developer | |
| Oscar Daudt Neto | Developer | |
| Oscar Gao | Developer | |
| Oscar Kenneth Albero Ingles | Developer | |
| Oscar Mordue | Developer | |
| Oscillatingcabbage | Developer | |
| Oshaberi | Developer | |
| Oshisaure | Developer | |
| Osiris Studios | Developer | |
| Oskar van Velden | Developer | |
| OskarDev | Developer | |
| OskarNystrom | Developer | |
| Oskarnik | Developer | |
| Oskarros | Developer | |