| 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 | |
| OniWasARacecarDriver | Developer | |
| Onigiri Seisakusho | Developer | |
| OniiOniiHopper | Developer | |
| Onildo Aguiar | Developer | |
| Onion Egg | Developer | |
| Onion Games | DeveloperPublisher | |
| Onion Samson | Developer | |
| Onion Software | Developer | |
| Onion Soup Interactive | DeveloperPublisher | |
| OnionBlaze | Developer | |
| OnionCorp | Developer | |
| Onionman77 | Developer | |
| OniriCorpe | Developer | |
| Oniria World | Developer | |
| Onirico Studios | Developer | |
| OnlineChungus | Developer | |
| OnlyGames | Developer | |
| OnlyPurpleRain | Developer | |
| OnlyShellder | Developer | |
| OnlySlightly | Developer | |
| Onlyjoy`s production | Developer | |
| Onslaught Supply Games | Developer | |
| Onstorm | Developer | |
| Onteca | DeveloperPublisher | |
| Onvisoc | Developer | |
| Onybr | Developer | |
| Onygox | Developer | |
| Onyric Productions | Developer | |
| Onyx Lute | Developer | |
| OnzaEstudio | Developer | |
| OofyDoodle | Developer | |
| Oogaphobia | Developer | |
| Ookami Kasumi | Developer | |
| Ooniversal | Developer | |
| Ooops24 | Developer | |
| Oopa Chaloopa | Developer | |
| Ooparts | Developer | |
| Oops | DeveloperPublisher | |
| Oops! | Developer | |
| Opa-Opa | Developer | |
| Opal Starlight | Developer | |
| Opalir | Developer | |
| Opaque Media | Developer | |
| Open Book | DeveloperPublisher | |
| Open Corporation | Developer | |
| Open Door Games | Developer | |
| Open House Games | Developer | |
| Open Lab Games | Developer | |
| Open Loop Games | Developer | |
| Open Sesame | Developer | |
| 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 | |