| OsmineeNitro | Developer | |
| Osmodium | Developer | |
| Osogato | Developer | |
| Osome Studio | DeveloperPublisher | |
| Ossete Games | Developer | |
| Ossian Studios | Developer | |
| Osspial | Developer | |
| Ostmark | Developer | |
| Ostrich Banditos | Developer | |
| Ostrze | Developer | |
| Otaara | Developer | |
| Otaboo | Developer | |
| Otadanku Productions | Developer | |
| Otaku Publishing Ltd. | Publisher | |
| OtakuHeroSama | Developer | |
| OtakuNerdGirl | Developer | |
| OtasGamer64 | Developer | |
| OtavioPastore | Developer | |
| Otay | Developer | |
| Otham | Developer | |
| Othelbark | Developer | |
| Other Ocean | DeveloperPublisher | |
| Other Ocean Interactive | DeveloperPublisher | |
| Other Tales Interactive | Developer | |
| Otherside Bay | Developer | |
| Otherside Entertainment | Developer | |
| Otherside Games | Developer | |
| OtherwiseAPPs | Developer | |
| OtisDev | Developer | |
| Otomate | Developer | |
| Otome Palace | Developer | |
| Otori Denshi | Developer | |
| Otozip | Developer | |
| Otsirave | Developer | |
| OtspIII | Developer | |
| Ottara Studio | Developer | |
| OtterWays | Developer | |
| Otterqueen | Developer | |
| Ouch Giver Games | Developer | |
| Ouch Mouse Studio | Developer | |
| OuchyBiteGuy | Developer | |
| Oudidon | Developer | |
| OuickTequila | Developer | |
| Ouili | Developer | |
| Ouji Akamine | Developer | |
| Oulsen | Developer | |
| Our Land Team | Developer | |
| Our Little Studio | Developer | |
| Ouro | Developer | |
| Ouro Eight | Developer | |
| OursBleu | Developer | |
| Ourson | Developer | |
| Out Of The Loop | Developer | |
| Out Of Tune Games | Developer | |
| Out of Box Games | Developer | |
| Out of Phayze | Developer | |
| Out of the Blue | Developer | |
| Out of the Park Developments | DeveloperPublisher | |
| OutLook Entertainment | Developer | |
| OutOfDate | Developer | |
| OutOut | Developer | |
| OutSide Directors Company | Developer | |
| Outback | Developer | |
| Outdoor Partners | Developer | |
| Outer Clouds Games | Developer | |
| OuterCloudStudio | Developer | |
| OuterRimStudios | Developer | |
| Outerlight | Developer | |
| Outerminds Alpha | Developer | |
| Outerminds Omega | Developer | |
| OutermostDev | Developer | |
| Outflank Strategy Wargames | Developer | |
| Outfrost | Developer | |
| Outgunned Games | Developer | |
| Outlander Games | Developer | |
| Outlands | Developer | |
| Outlaw Software | Developer | |
| Outlaws Studio | Developer | |
| Outlook Entertainment | Developer | |
| Output Games | Developer | |
| Outrage Entertainment | Developer | |
| Outrage Games | Developer | |
| Outrigger Koubou | Publisher | |
| Outright Games | DeveloperPublisher | |
| Outsider Games | DeveloperPublisher | |
| Outsider Team | Developer | |
| Outspark | Publisher | |
| OutspokenWalnut Studios | Developer | |
| OveAhlman | Developer | |
| OveR-Software | Developer | |
| Over Cooked Code Games | Developer | |
| Over Fence | Developer | |
| Over The Top Games | Developer | |
| Over the Moon | DeveloperPublisher | |
| Over the Rainbow Studios | Developer | |
| Over the Top Games | Developer | |
| Over-9000 | Developer | |
| OverGamez | DeveloperPublisher | |
| OverPowered Team | Developer | |
| OverTune | Developer | |
| Overboundn | Developer | |
| Overflow | Developer | |
| Overgame Studio | DeveloperPublisher | |
| Overgrown | Developer | |
| Overhead Games | Developer | |
| Overkill Software | Developer | |
| Overmax Studios | Developer | |
| Overpowered Games | Developer | |
| Overseas | Developer | |
| Overtired_FOX | Developer | |
| Overtold | Developer | |
| OverwaterGames Official | Developer | |
| OverworkedStudios | Developer | |
| Overworks | DeveloperPublisher | |
| Overworld Galaxy | Developer | |
| Ovid Works | Developer | |
| Ovidiu Barac | Developer | |
| Ovine by Design | DeveloperPublisher | |
| Ovosonico | Developer | |
| OwT | Developer | |
| Owen Ayers | Developer | |
| Owen Brunette | Developer | |
| Owen Deery | Developer | |
| Owen Emry | Developer | |
| Owen Heinz | Developer | |
| OwenM45 | Developer | |
| OwenSenior | Developer | |
| OwenTheBell | Developer | |
| Owhno | Developer | |
| Owl Nest Studios | Developer | |
| Owl Sanctuary Studios | Developer | |
| OwlTechInteractive | Developer | |
| OwllCraft | Developer | |
| Owlor | Developer | |
| OwlyCode | Developer | |
| Owned by Gravity | Developer | |
| Ox Flame Game Dev | Developer | |
| Oxbane Project | Developer | |
| Oxeye Game Studio | DeveloperPublisher | |
| Oxford Digital Enterprises | Developer | |
| Oxford Digital Enterprises (Kevin R. Ayre) | Developer | |
| Oxford Digital Enterprises (Michael Fox) | Developer | |
| Oxford Digital Enterprises Ltd. | Developer | |
| Oxford Mobius | Developer | |
| Oxford Softworks | DeveloperPublisher | |
| OxfordLucrezia | Developer | |
| OxiWan | Developer | |
| Oxidental Games | Developer | |
| Oxonian Games | Developer | |
| Oxygen Games | DeveloperPublisher | |