| Name | Types | |
|---|---|---|
| O-A-K | Developer | |
| O-Games | DeveloperPublisher | |
| O-GamesNA | Publisher | |
| O-TWO | Developer | |
| O.Y.G | Developer | |
| O2 Games | DeveloperPublisher | |
| O2 Online Entertainment | DeveloperPublisher | |
| O3 Entertainment | Publisher | |
| O3 EntertainmentNA | Publisher | |
| O3 Games | Developer | |
| OAB | Developer | |
| OAI | Developer | |
| OCC | DeveloperPublisher | |
| ODIAX | Developer | |
| ODIN Software | DeveloperPublisher | |
| ODS | Developer | |
| ODenis Studios | Developer | |
| OG International Ltd | Publisher | |
| OG-M | Developer | |
| OGD | Developer | |
| OGDarkWolfie | Developer | |
| OGPlanet | Publisher | |
| OGUREC APPS | Developer | |
| OG_Hamster | Developer | |
| OHISASHI | Developer | |
| OIDyre | Developer | |
| OKSoft | Developer | |
| OKYOGames | Developer | |
| OLEANDER GARDEN | Developer | |
| OLEANDRIN | Developer | |
| OLGA | Developer | |
| OLKO | Developer | |
| OLM | Developer | |
| OM M | Developer | |
| OMGWTFGAMES!1!! | Developer | |
| OMI Games | Developer | |
| OMK Software | DeveloperPublisher | |
| OMOCAT | Developer | |
| OMega Mikotto | Developer | |
| ON3D Studios | Developer | |
| ONCGM | Developer | |
| OOO Valnat | Publisher | |
| OOTP Developments | DeveloperPublisher | |
| OP Games | Developer | |
| OPENsupermen | Developer | |
| OPQAM | Developer | |
| OPeNBooK | DeveloperPublisher | |
| OPeNBooK Co., Ltd. | Developer | |
| OPeNBooK9003 | DeveloperPublisher | |
| ORIGIN Systems | Developer | |
| ORT Software | DeveloperPublisher | |
| OSC | Publisher | |
| OSP Enr. | DeveloperPublisher | |
| OSR | Developer | |
| OSTA | Developer | |
| OSome Studios | Developer | |
| OTGDev | Developer | |
| OTLGame | Developer | |
| OUAT Entertainment | Developer | |
| OVERBOY ツ | Developer | |
| OX | Developer | |
| OXTORD games | Developer | |
| OXiAB Game Studio | DeveloperPublisher | |
| OZ Club | DeveloperPublisher | |
| Oachkatzl | Developer | |
| OakLeafGames | Developer | |
| OakiDev | Developer | |
| Oakleaff | Developer | |
| OakleyBlade | Developer | |
| Oaks Soft | Developer | |
| Oaktown Studios | Developer | |
| Oaky Games | Developer | |
| Oasis Games | Publisher | |
| Oasis Plus | Developer | |
| OatCube | Developer | |
| OatsCreative | Developer | |
| Obaasan Games | Developer | |
| ObakeNekoWorks | Developer | |
| Obelix | Developer | |
| ObentoMori | Developer | |
| Obeonix | Developer | |
| Oberon Media | Publisher | |
| Obi-Gun Kenobi | Developer | |
| ObiSam2017 | Developer | |
| Object Software | Developer | |
| Object Software Limited | Developer | |
| Objectif 3D | Developer | |
| Objectively Big | Developer | |
| Obo Games | Developer | |
| Obscurata_Studios | Developer | |
| Obscure Tales | Developer | |
| Obsessive Science Games | Developer | |
| Obside | Developer | |
| Obsidian Entertainment | Developer | |
| ObsidianBlk | Developer | |
| Obsyds's Game | Developer | |
| Obunsha | DeveloperPublisher | |
| OcO | Developer | |
| Ocarina System | Developer | |
| OccOcGame | Developer |