| Omega Mikotto | DeveloperPublisher | |
| Omega Products | DeveloperPublisher | |
| Omega Software | DeveloperPublisher | |
| Omega-8 Fatty Acid | Developer | |
| OmegaMagnus | Developer | |
| OmegaSoft | Developer | |
| Omega_Amminus | Developer | |
| Omegavision | Developer | |
| Omehu | Developer | |
| Omelette Dev | Developer | |
| OmerBhatti | Developer | |
| Omergosh | Developer | |
| Omich | Developer | |
| OmicronPerson | Developer | |
| OmidGam3r | Developer | |
| Omiya Games | Developer | |
| Omiya Soft | Developer | |
| OmiyaSoft | Developer | |
| Ommageden | Developer | |
| OmmuGames | Developer | |
| Omni Creative | Developer | |
| Omni System Limited | Developer | |
| Omni Systems | Developer | |
| Omni Systems Limited | Developer | |
| Omnipotent Potato | Developer | |
| Omnitrend Software | DeveloperPublisher | |
| Omnitron Studios | Developer | |
| OmochiGames | Developer | |
| Omori Electric | Publisher | |
| Omori Electric Co., Ltd. | Publisher | |
| Omori Electric Company | Developer | |
| Omoro Electric Company | Developer | |
| Omulette | Developer | |
| On Demand | DeveloperPublisher | |
| On The Metal | Developer | |
| On-Line PLC | Publisher | |
| On-Line Systems | DeveloperPublisher | |
| OnSkull Development | Publisher | |
| OnSkull Games | Developer | |
| Onaisuo | Developer | |
| Onanagro Studio | Developer | |
| Onanim Studio | Developer | |
| OndKaar | Developer | |
| OndeadGames | Developer | |
| Ondrej Trhon | Developer | |
| One Aperture | Developer | |
| One Bit Beyond | Developer | |
| One Bit Studio | Developer | |
| One Byte | Developer | |
| One Day Code | Developer | |
| One Dove Above | Developer | |
| One Fox | Developer | |
| One Gruel Studio | Developer | |
| One Guy Games | Developer | |
| One Guy Studio | Developer | |
| One Hand Free Studios | Developer | |
| One Happy Giant | Developer | |
| One Hit Combo Studio | Developer | |
| One Man Army FL | Developer | |
| One More Dream Studios | Developer | |
| One More Level | Developer | |
| One True Game Studios | Developer | |
| One Up Plus Entertainment | Developer | |
| One Week Productions (Colin Swinbourne, Kevin Parker, Martin Severn, Andrew Severn, Mike Brown) | Developer | |
| One Week Productions (Kevin Parker, Colin Swinbourne, Martin Severn, Andrew Severn) | Developer | |
| One Week Productions (Kevin Parker, Colin Swinbourne, Martin Severn, Andrew Severn, Mike Brown) | Developer | |
| One Week Productions (Kevin Parker, Martin Severn) | Developer | |
| One Wolf | Developer | |
| One or Eight | DeveloperPublisher | |
| One-bit Punch | Developer | |
| OneDollar | Developer | |
| OneEgg42 | Developer | |
| OneGameDad | Developer | |
| OneGuyGames | Developer | |
| OneHundredThousand | Developer | |
| OneManArmy | Developer | |
| OneManArmyStudios | Developer | |
| OneManBand | Developer | |
| OneMoreLevel | Developer | |
| OneNine Studios | Developer | |
| OnePieceOnlineRPG | Developer | |
| OnePixelMonster | Developer | |
| OneSecretPseudo | Developer | |
| OneShark | Developer | |
| OneThatEatYou | Developer | |
| OneZeroDev | Developer | |
| Onebraverobot | Developer | |
| OneginIII | Developer | |
| Oneiric Worlds | Developer | |
| Oneirocritica Games | Developer | |
| Oneiromancer | Developer | |
| OneorEight | Developer | |
| Onerat Games | Developer | |
| OneratGames | Developer | |
| Onesong Partners | Developer | |
| Onevision | DeveloperPublisher | |
| Onevision Games | DeveloperPublisher | |
| Ongakukan | DeveloperPublisher | |
| OnheXon Entertainment | Developer | |
| Oni Elite | Developer | |