| O2Jam | music video game | — | Microsoft Windows |
| Obey Me | — | — | Microsoft Windows |
| Objection! | — | — | DOS |
| Obliteracy | — | — | Microsoft Windows, macOS |
| Obliterator | — | David Whittaker | DOS, ZX Spectrum, Amstrad CPC, Atari ST |
| OboStar | — | — | Microsoft Windows |
| Obscura | visual novel | — | Microsoft Windows, macOS, Linux |
| Obscurant | adventure video game, action game, action-adventure game, role-playing video game | — | Microsoft Windows, macOS, Linux |
| Observer | science fiction video game, cyberpunk video game, psychological horror fiction | Bloober Team | Microsoft Windows, macOS, Linux |
| Obsessive Revenge | casual game, action game, strategy video game | — | Microsoft Windows |
| Ocean Trader | business simulation game | — | Microsoft Windows |
| Ocean's Heart | action role-playing game | — | Microsoft Windows |
| Oceanhorn 2: Knights of the Lost Realm | action role-playing game | — | Microsoft Windows |
| Oceanhorn: Chronos Dungeon | — | Cornfox & Bros. | macOS |
| Oceanhorn: Monster of Uncharted Seas | action-adventure game | Cornfox & Bros. | Microsoft Windows, macOS, PlayStation Vita, Android |
| Oceanum Mortis | casual game, adventure video game, role-playing video game | — | Microsoft Windows |
| Octocopter: Double Or Squids | — | TACS Games | — |
| Octocopter: Super Sub Squid Escape | — | — | Wii U |
| Octodad: Dadliest Catch+ | — | — | — |
| Octogeddon | — | — | Microsoft Windows |
| Octopath Traveler | role-playing video game | Square Enix, Acquire | Microsoft Windows |
| Ode to Ramon 3 | — | Team Hoi | Amiga 1200 |
| Odyssey | — | Audiogenic | — |
| Off-Road Farming | simulation video game, racing video game | Enoops | Microsoft Windows |
| Offendron Warrior | action game | — | Microsoft Windows |
| OfficeBots: Reality Bytes [VR] | — | — | Microsoft Windows |
| OGgolf | sports video game | — | Microsoft Windows, Linux |
| Ogre | — | Auroch Digital | Microsoft Windows, macOS |
| Ogre Battle: The March of the Black Queen | tactical role-playing game, real-time tactics | Quest | Super Nintendo Entertainment System, Wii U |
| Oh No! More Lemmings | — | — | SAM Coupé, Microsoft Windows, DOS, Acorn Archimedes, Atari ST |
| OH! RPG! | casual game, adventure video game, strategy video game, role-playing video game | — | Microsoft Windows |
| Ohayou! Beginner's Japanese | casual game | — | Microsoft Windows, Wii U |
| Oik 2 | casual game | Crew Lab | Microsoft Windows |
| Oik 3 | casual game | Crew Lab | Microsoft Windows |
| Oik Memory 2 | casual game | Crew Lab | Microsoft Windows |
| Ojamajo Adventure: Naisho no Mahou | visual novel | Amada Printing | Microsoft Windows |
| Okamoto Ayako no Match Play Golf | golf video game | — | Othello Multivision |
| Okiba ga Nai! | visual novel | — | Microsoft Windows |
| Okinawa Rush | — | Sokaikan | Microsoft Windows, Xbox Series X and Series S |
| Okitsune Summer: Natsu Gasshuku Onna no Ko Tsuki | eroge, visual novel | — | Microsoft Windows |
| Oknytt | adventure video game | — | Microsoft Windows |
| Old Factory | — | — | Microsoft Windows |
| Old School Horror Game : Bright Day | — | — | Microsoft Windows |
| Olho Adventure | adventure video game | — | Microsoft Windows |
| Olija | adventure video game, action game, action-adventure game, platform game | Skeleton Crew Studio, Thomas Olsson | Microsoft Windows |
| Ollie & Bollie: Outdoor Estate | casual game | Sokpop Collective | Microsoft Windows, macOS |
| Olympus Griddlers | — | Creobit | Microsoft Windows |
| Olé Toro | — | Dinamic Software | ZX Spectrum, Amstrad CPC |
| Omae Wa Mou Shindeiru | casual game | Laush Studio | Microsoft Windows |
| Omega Agent | — | Fireproof Studios | Microsoft Windows |
| Omega Labyrinth | roguelike | — | PlayStation Vita |
| Omega Labyrinth Z | roguelike | Matrix Software | PlayStation Vita |
| OMEGA: The Beginning - Episode 1 | adventure video game | — | Microsoft Windows, macOS, Linux |
| OMFG: One Million Fatal Guns | action game | — | Microsoft Windows |
| OMG: One Million Guns | action game | — | Microsoft Windows |
| On Board 4 PC | — | — | Microsoft Windows |
| On Space | — | — | Microsoft Windows |
| On the Peril of Parrots | casual game | — | Microsoft Windows |
| Once in Flowerlake | casual game, simulation video game, adventure video game | — | Microsoft Windows |
| Once upon a time in the Gold Rush VR: shoot and ride | casual game | William at Oxford | Microsoft Windows |
| Onde | puzzle video game, platform game | — | Microsoft Windows |
| One By One | casual game, adventure video game, role-playing video game | Rosa Special Studio | Microsoft Windows |
| One Day | casual game, action game | — | Microsoft Windows |
| One Dreamer | adventure video game | — | Microsoft Windows, macOS, Linux |
| One Finger Death Punch 2 | action game | Silver Dollar Games | Microsoft Windows |
| One Hell of a Ride | — | — | Xbox Series X and Series S |
| One Last Memory | simulation video game, adventure video game, action game, action-adventure game | EpiXR Games | Microsoft Windows |
| One Night At Barneys 3 | simulation video game, strategy video game | — | Microsoft Windows |
| One Piece: Pirate Warriors 2 | crowd-combat video game, action-adventure game | Koei Tecmo Holdings | PlayStation 3, PlayStation Vita |
| One Piece: Unlimited Cruise 2 – Awakening of a Hero | action-adventure game | Ganbarion | Wii |
| One True Cuddle | adventure video game, action game, action-adventure game | — | Microsoft Windows |
| One True Hero | platform game | Rat Cliff Games | Microsoft Windows, Xbox Series X and Series S |
| One Week By Car | — | — | Microsoft Windows |
| One: Kagayaku Kisetsu e | eroge, drama anime and manga, fantasy anime and manga, romance anime and manga, school anime and manga, visual novel, harem | Tactics | Microsoft Windows, Java virtual machine |
| Onechanbara Z: Kagura with Nonono! | — | — | — |
| OneJump | casual game | — | Microsoft Windows |
| Oneons: Prisoners | casual game | — | Microsoft Windows, Linux |
| Onimusha 2: Samurai's Destiny | dark fantasy video game, hack and slash, action-adventure game | Capcom | PlayStation 3 |
| Onimusha: Dawn of Dreams | dark fantasy video game, hack and slash, action-adventure game | Capcom | — |
| Only Mine | — | — | Microsoft Windows |
| Only Up! | casual game, simulation video game, action game, platform game, massively multiplayer online game | — | Microsoft Windows |
| Only You -Re Cross- | — | AliceSoft | — |
| OnlyFap Simulator 3 | casual game, simulation video game, adventure video game | — | Microsoft Windows |
| OnlySociety: Dawn | casual game, adventure video game, action game, action-adventure game | — | Microsoft Windows |
| Onmyoji in the Otherworld: Sayaka's Story | adventure video game, strategy video game | Debonosu Works | Microsoft Windows |
| Onrush | — | Codemasters | — |
| Onryo | casual game, simulation video game, adventure video game, action game, action-adventure game | Chilla's Art | Microsoft Windows |
| Onyx | casual game, adventure video game, role-playing video game | — | Microsoft Windows |
| Oops!!! Puzzles!!! | — | — | Microsoft Windows |
| Opaline | casual game, adventure video game, role-playing video game | Warfare Studios | Microsoft Windows |
| Openciv | strategic game | — | DOS |
| Operation C | run and gun | Konami | Game Boy |
| Operation Creature Feature | — | London Studio | PlayStation 3 |
| Operation Flashpoint: Dragon Rising | — | Codemasters | PlayStation 3, Microsoft Windows |
| Operation Flashpoint: Resistance | — | Bohemia Interactive | Microsoft Windows |
| Operation Tango | casual game, adventure video game | Clever Plays | Microsoft Windows, Xbox Series X and Series S |
| Operation Thunder | action game | — | Microsoft Windows, Linux |
| Operation: Black Mesa | first-person shooter | — | Microsoft Windows |
| Oppai Slider 2 | eroge, visual novel | Illusion | Microsoft Windows |
| Oppaidius: Tropical Cruise! | visual novel | — | Microsoft Windows, macOS, Linux |
| Optica | casual game, strategy video game | — | Microsoft Windows, macOS |
| Orangeblood | role-playing video game | — | Microsoft Windows |
| Orbals | adventure video game, strategy video game | FarSight Studios | Microsoft Windows |
| Orbo's Odyssey | platform game | Feverdream Softworks | Microsoft Windows |
| Orborun | rolling platformer, platform game | — | Microsoft Windows, macOS, Linux |
| OrcCraft | — | — | Microsoft Windows |
| Organ Quarter | adventure video game | — | Microsoft Windows |
| Organs Please | simulation video game, strategy video game, role-playing video game | — | Microsoft Windows |
| Original Journey | adventure video game, action game, action-adventure game, role-playing video game | — | Microsoft Windows, macOS |
| Orion Burger | — | Sanctuary Woods | Classic Mac OS, DOS |
| Ork | — | — | Atari ST |
| Orpheus: Fall For Eurydice | adventure video game, action game, action-adventure game | — | Microsoft Windows, macOS, Linux |
| Orwell's Animal Farm | — | Nerial | Microsoft Windows, macOS, Android |
| Oscar & Gems: Puzzle Quest | casual game, adventure video game, action game, action-adventure game | Dnovel, FreeAnimals_Software | Microsoft Windows |
| OscarWildeCard | casual game, strategy video game | — | Microsoft Windows, macOS, Linux |
| Oscillate | simulation video game, adventure video game, racing video game, sports video game | — | Microsoft Windows |
| OtherSoul | casual game, action game | — | Microsoft Windows |
| Otome ga Tsumugu Koi no Canvas | eroge, visual novel | Ensemble | Microsoft Windows |
| OU | adventure video game | — | Microsoft Windows |
| Oumagatoki | — | — | — |
| Our Journeys 2 | — | — | Microsoft Windows |
| Our Personal Space | visual novel | Metasepia Games | Microsoft Windows, macOS, Linux, Android |
| Out of Order | — | — | Microsoft Windows, macOS, Linux |
| Outcore: Desktop Adventure | adventure video game, puzzle video game | — | Microsoft Windows |
| Outer Terror | action-adventure game | — | Microsoft Windows |
| OUTLANDER | adventure video game, role-playing video game | — | Microsoft Windows |
| Outlander | — | Mindscape | Super Nintendo Entertainment System |
| Outlaw Golf 2 | golf video game | — | — |
| Outpost | construction and management simulation, science fiction video game | Q494614 | Classic Mac OS, Microsoft Windows |
| OutRun 2006: Coast 2 Coast | — | Sumo Digital | Microsoft Windows, PlayStation Portable, Nintendo GameCube |
| Over The Cloud : Lost Planet | — | — | Microsoft Windows |
| Overcoming Obstacles | platform game | — | Microsoft Windows |
| Overdrive | — | Superior Software | Acorn Electron |
| Overgrown | action game | — | Microsoft Windows |
| Overlanders | racing video game | — | Microsoft Windows |
| Overlord II | — | Electronic Arts, Triumph Studios | PlayStation 3, Microsoft Windows, macOS, Linux |
| Overlord: Escape from Nazarick | Metroidvania | — | Microsoft Windows |
| Overlord: Minions | — | Climax Studios | Nintendo DS |
| Overlord: Raising Hell | — | Triumph Studios | PlayStation 3, Microsoft Windows, macOS |
| Override 2: Super Mech League | fighting game | Q104628690 | Microsoft Windows, Xbox Series X and Series S |
| Overrogue | roguelike deck-building game | Exe-Create | Microsoft Windows, Android, Xbox Series X and Series S |
| OverWall | action game, strategy video game | — | Microsoft Windows |
| Oxenfree | thriller, adventure video game, graphic adventure game, science fiction video game, coming-of-age fiction, supernatural fiction | Night School Studio | Microsoft Windows, macOS, Linux, Android |
| Oylinder | casual game | — | Microsoft Windows |
| Ozzy & Drix | platform game | — | — |
| Önskejakten | — | — | web browser |
| Ōzoku | eroge, simulation video game, visual novel | Softhouse Chara | Microsoft Windows |