| O'Leary Manager 2000 | 2000 | — | Game Boy Color |
| O.D.T. | 1998 | SCE Studio Liverpool | PlayStation, Microsoft Windows |
| O.R.B: Off-World Resource Base | 2002 | Strategy First | Microsoft Windows |
| Obitus | 1991 | — | Commodore Amiga, DOS, Super Nintendo Entertainment System, Atari ST |
| Obliterator | 1988 | David Whittaker | Commodore Amiga, DOS, ZX Spectrum, Amstrad CPC, Atari ST |
| Observation | 2019 | Screen Burn | Microsoft Windows, PlayStation 4 |
| Obsidian | 1986 | Tony Warriner | Amstrad CPC |
| Octamari Rescue | 2016 | Sumo Digital | Microsoft Windows, macOS |
| Octapolis | 1987 | Jukka Tapanimäki | Commodore 64 |
| Oddworld: Hand of Odd | — | Just Add Water | — |
| Oddworld: New 'n' Tasty! | 2015 | Oddworld Inhabitants, Just Add Water | PlayStation 3, Xbox One, Microsoft Windows, macOS, PlayStation Vita, Nintendo Switch, Linux, iOS, PlayStation 4, Wii U, Android |
| Ode to Ramon 1 | 1988 | Team Hoi | Amiga 1200 |
| Ode to Ramon 2 | 1989 | Team Hoi | Amiga 1200 |
| Ode to Ramon 3 | 1994 | Team Hoi | Amiga 1200 |
| Odinfall | 2025 | — | Microsoft Windows |
| Odyssey | 1995 | Audiogenic | Commodore Amiga |
| Off Road | 2008 | Razorworks | PlayStation 2, Microsoft Windows, personal computer, PlayStation Portable, Wii |
| Off-Road Redneck Racing | 2001 | — | Microsoft Windows |
| Ogre | 2017 | Auroch Digital | Microsoft Windows, macOS |
| Oh Mummy | 1984 | — | ZX Spectrum, Amstrad CPC, iOS, MSX |
| Oh No! More Lemmings | 1991 | Rockstar North | Commodore Amiga, SAM Coupé, Microsoft Windows, DOS, Acorn Archimedes, Atari ST |
| Okinawa Rush | 2021 | Sokaikan | Xbox One, Microsoft Windows, Nintendo Switch, PlayStation 4, Xbox Series X and Series S |
| Old School RuneScape | 2013 | Jagex | Microsoft Windows, macOS, Linux, iOS, Android |
| Olli and Lissa | 1987 | — | ZX Spectrum, Amstrad CPC, Commodore 64 |
| OlliOlli | 2014 | Roll7 | Microsoft Windows, macOS, PlayStation Vita, Linux, PlayStation 4, Wii U, Android |
| OlliOlli World | 2022 | Roll7 | Xbox One, Microsoft Windows, Nintendo Switch, PlayStation 4, PlayStation 5, Xbox Series X and Series S |
| OlliOlli2 | 2015 | Roll7 | Xbox One, Microsoft Windows, macOS, PlayStation Vita, Linux, PlayStation 4, Android |
| Olympic Gold | 1992 | Tiertex Design Studios | Sega Genesis/Mega Drive, Sega Master System, Game Gear |
| Olympic Soccer | 1996 | — | PlayStation, Sega Saturn |
| Olympic Summer Games | 1996 | Silicon Dreams Studios, Tiertex Design Studios | Sega Genesis/Mega Drive, PlayStation, Super Nintendo Entertainment System, Game Boy, 3DO |
| Olympus Griddlers | 2020 | Creobit | Microsoft Windows |
| Olé Toro | — | Dinamic Software | ZX Spectrum, Amstrad CPC |
| Omega Agent | 2016 | Fireproof Studios | Microsoft Windows |
| OMG-Z | 2011 | Laughing Jackal | PlayStation 3, Microsoft Windows, PlayStation Vita, Nintendo Switch |
| One Man and His Droid | 1985 | — | ZX Spectrum, Amstrad CPC, Commodore 64 |
| One Move Away | 2026 | — | Nintendo Switch 2, Microsoft Windows, Nintendo Switch, PlayStation 5, Xbox Series X and Series S |
| Onrush | 2018 | Codemasters | Xbox One, PlayStation 4 |
| Onslaught | 1989 | realm | Sega Genesis/Mega Drive, DOS, iOS |
| Onward | 2016 | Downpour Interactive | Microsoft Windows |
| Operation Air Assault | 2004 | Naps Team | PlayStation 2 |
| Operation Creature Feature | 2007 | London Studio | PlayStation 3 |
| Operation Flashpoint: Cold War Crisis | 2001 | Bohemia Interactive | Xbox, Microsoft Windows, macOS, Linux |
| Operation Flashpoint: Dragon Rising | 2009 | Codemasters | PlayStation 3, Microsoft Windows, Xbox 360 |
| Operation Flashpoint: Red Hammer | — | — | Microsoft Windows |
| Operation Flashpoint: Red River | 2011 | Codemasters | PlayStation 3, Microsoft Windows, Xbox 360 |
| Operation Flashpoint: Resistance | 2002 | Bohemia Interactive | Microsoft Windows |
| Operation: New Earth | 2016 | Hunted Cow Studios | Microsoft Windows, macOS |
| Operation: Vietnam | 2007 | — | Nintendo DS |
| Orbiter | 2000 | — | Microsoft Windows |
| Orc Attack | 1983 | Aackosoft, Creative Sparks | ZX Spectrum, Commodore 64 |
| Order of Battle: World War II | 2015 | The Aristocrats | Microsoft Windows, macOS |
| Oriental Empires | 2016 | Shining Pixel Studios | Microsoft Windows |
| Original War | 2001 | Altar Games | Microsoft Windows |
| Orion Burger | 1996 | Sanctuary Woods | Classic Mac OS, DOS |
| Ork | 1991 | — | Commodore Amiga, Atari ST |
| Orwell's Animal Farm | 2020 | Nerial | Microsoft Windows, macOS, iOS, iPadOS, Android |
| Oscar | 1993 | Flair Software | Commodore Amiga, DOS, Super Nintendo Entertainment System, Amiga CD32 |
| Ostron | 1983 | Andrew Glaister | ZX Spectrum |
| Our World Is Ended | 2017 | Red Entertainment | Microsoft Windows, PlayStation Vita, Nintendo Switch, PlayStation 4 |
| Out of Order | 2003 | — | Microsoft Windows, macOS, Linux |
| Out of the Shadows | 1984 | — | ZX Spectrum |
| Out Run Europa | 1991 | Acclaim Studios London | Commodore Amiga, Sega Master System, ZX Spectrum, Amstrad CPC, Atari ST, Commodore 64 |
| Out to Lunch | 1993 | Mindscape | Commodore Amiga, Super Nintendo Entertainment System |
| Outbound | 2026 | Square Glade Games | Nintendo Switch 2, Xbox One, Microsoft Windows, Nintendo Switch, PlayStation 5, Xbox Series X and Series S |
| Outlander | 1993 | Mindscape | Sega Genesis/Mega Drive, Super Nintendo Entertainment System |
| Outlaws | 1985 | Ultimate Play the Game | Commodore 64 |
| OutRun 2 | 2003 | Sega AM2 | PlayStation 2, Xbox, Nintendo GameCube, arcade video game machine |
| OutRun 2006: Coast 2 Coast | 2006 | Sumo Digital | PlayStation 2, Xbox, Microsoft Windows, PlayStation Portable, Nintendo GameCube |
| OutRun Online Arcade | 2009 | Sumo Digital | PlayStation 3, Xbox 360 |
| Outward | 2019 | Nine Dots Studio | Microsoft Windows |
| Overboard! | 2021 | Inkle | Microsoft Windows, macOS, Nintendo Switch, iOS |
| Overboard! | 1997 | SCE Studio Liverpool | PlayStation, Microsoft Windows |
| Overcooked | 2016 | Ghost Town Games | Microsoft Windows |
| Overcooked 2 | 2018 | — | Nintendo Switch 2, Xbox One, Microsoft Windows, macOS, Nintendo Switch, Linux, PlayStation 4 |
| Overcooked! All You Can Eat | 2021 | Team17, Ghost Town Games | Xbox One, Microsoft Windows, Nintendo Switch, PlayStation 4, PlayStation 5, Xbox Series X and Series S |
| Overcrowd: A Commute 'Em Up | 2019 | SquarePlay Games | Microsoft Windows |
| Overdrive | 1993 | — | Commodore Amiga, MS-DOS |
| Overdrive | 1984 | Superior Software | Acorn Electron |
| Overkill | 1992 | — | DOS |
| Overlord | 2007 | Electronic Arts, Triumph Studios | PlayStation 3, Microsoft Windows, macOS, Linux, Xbox 360 |
| Overlord | 1994 | Rowan Software | Commodore Amiga, DOS |
| Overlord II | 2009 | Electronic Arts, Triumph Studios | PlayStation 3, Microsoft Windows, macOS, Linux, Xbox 360 |
| Overlord: Dark Legend | 2009 | Climax Studios | Wii |
| Overlord: Fellowship of Evil | 2015 | Codemasters | Xbox One, Microsoft Windows, PlayStation 4 |
| Overlord: Minions | 2009 | Climax Studios | Nintendo DS |
| Overlord: Raising Hell | 2008 | Triumph Studios | PlayStation 3, Microsoft Windows, macOS, Xbox 360 |
| Overruled! | 2015 | — | Microsoft Windows, macOS, Linux |
| OXO | 1952 | Sandy Douglas | Electronic Delay Storage Automatic Calculator |