| 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 |