| title | developer | year | platform | | pop. |
|---|
| ObsCure | Hydravision | 2004 | Windows | action/reflex Fantasy | image | | label | |
| ObsCure | Hydravision | 2004 | Xbox | action/reflex Fantasy | image | | label | |
| Odin Sphere | Vanillaware | 2007 | PS2 | action/reflex beat 'em up role-play Fantasy | image | subject | label | |
| Odin Sphere Leifthrasir | Vanillaware | 2016 | PS4 | action/reflex beat 'em up role-play Fantasy | image | subject | label | |
| Odin Sphere Leifthrasir | Vanillaware | 2016 | PS3 | action/reflex beat 'em up role-play Fantasy | image | | label | |
| Odin Sphere Leifthrasir | Vanillaware | 2016 | PS Vita | action/reflex beat 'em up role-play Fantasy | image | | label | |
| Of Orcs and Men | Cyanide;Spiders | 2012 | Windows | action/reflex Fantasy | image | | label | |
| Ogre Hills | NewPlanet Productions | 2013 | Linux | role-play | | | label | |
| Omerta: City of Gangsters | Haemimont Games | 2013 | Windows | simulation role-play turn-based Historical | image | subject | label | |
| One Heart | 2 Hit Studio | 2014 | Linux | fighting platformer | | | label | |
| OpenQuartz | Open Quartz Team | 2001 | Linux | shooter | | subject | label | |
| OpenQuartz | Open Quartz Team | ? | BeOS | shooter | | | label | |
| OQPlus | author | ? | Linux | shooter | | | label | |
| OQPlus | ? | ? | BeOS | shooter | | | label | |
| Orcs Must Die! | Robot Entertainment | 2011 | Windows | action/reflex strategy Fantasy | image | subject | label | |
| Orcs Must Die! | Robot Entertainment | 2011 | X360 | action/reflex strategy Fantasy | image | | label | |
| Orcs Must Die! | Robot Entertainment | 2014 | Linux | action/reflex strategy Fantasy | | | label | |
| Orczz | Legenda Studios | 2013 | Linux | strategy Fantasy | | | label | |
| Order of the Thorne 2 | Infamous Adventures | TBA | Linux | adventure Fantasy | | | label | |
| Order of the Thorne: The King’s Challenge | Infamous Adventures | 2015 | Linux | adventure Fantasy | | | label | |
| ORiGiN | author | 2007 | Linux | action/reflex adventure Fantasy | image | subject | label | |
| OS2Moria | author | 1992 | OS/2 | role-play Fantasy | | | label | |
| Other Life | Other-Life | 2013 | Linux | role-play Fantasy | | | label | |
| Out of Hell | author | 2008 | Linux | Horror | | | label | |
| Overgrowth | Wolfire Games | 2017 | Windows | action/reflex adventure Fantasy | image | | label | |
| Overgrowth | Wolfire Games | 2017 | Mac OS X | action/reflex adventure Fantasy | | | label | |
| Overgrowth | Wolfire Games | 2017 | Linux | action/reflex adventure Fantasy | | | label | |
| Overlord | Triumph Studios | 2007 | Windows | action/reflex adventure Fantasy | image | subject | label | |
| Overlord | Triumph Studios | 2007 | X360 | action/reflex adventure Fantasy | image | | label | |
| Overlord | Triumph Studios | 2007 | Linux | action/reflex adventure Fantasy | | | label | |
| Overlord II | Triumph Studios | 2009 | Windows | action/reflex adventure Fantasy | image | | label | |
| Overlord II | Triumph Studios | 2009 | X360 | action/reflex adventure Fantasy | image | | label | |
| Overlord II | Triumph Studios | 2009 | PS3 | action/reflex adventure Fantasy | image | | label | |
| Overlord II | Triumph Studios | 2009 | Linux | action/reflex adventure Fantasy | | | label | |
| Overlord: Raising Hell | Triumph Studios | 2008 | Windows | action/reflex adventure Fantasy | image | | label | |
| Overlord: Raising Hell | Triumph Studios | 2008 | PS3 | action/reflex adventure Fantasy | image | | label | |
| Overlord: Raising Hell | Triumph Studios | 2008 | X360 | action/reflex adventure Fantasy | image | | label | |
| Overlord: Raising Hell | Triumph Studios;Virtual Programming | 2016 | Linux | action/reflex adventure Fantasy | | | label | |
