| title | developer | year | platform | | pop. |
|---|
| Oberon 69 | Diabolic Software | 1990 | ZX Spectrum | platformer shooter Science Fiction | image | | label | |
| Oberon 69 | Diabolic Software | 1990 | Amiga | platformer shooter Science Fiction | image | | label | |
| Oberon 69 | Diabolic Software | 1990 | Amstrad CPC | platformer shooter Science Fiction | image | | label | |
| Oberon 69 | Diabolic Software | 1990 | MSX | platformer shooter Science Fiction | image | | label | |
| Oberon 69 | Diabolic Software | 1990 | MS-DOS | platformer shooter Science Fiction | image | | label | |
| ObsCure | Hydravision | 2004 | Windows | action/reflex Fantasy | image | | label | |
| ObsCure | Hydravision | 2004 | Xbox | action/reflex Fantasy | image | | label | |
| Omerta: City of Gangsters | Haemimont Games | 2013 | Windows | simulation role-play turn-based Historical | image | subject | label | |
| Oni | Bungie Studios | 2001 | Windows | action/reflex beat 'em up shooter Manga | image | | label | |
| Oni | Rockstar Games | 2001 | PS2 | action/reflex beat 'em up Manga | image | subject | label | |
| Oniken | author | 2012 | Linux | beat 'em up platformer shooter | | | label | |
| Oniken | author | 2012 | Windows | beat 'em up platformer shooter | | | label | |
| Open Surge | Open Sonic Team | 2008 | Linux | platformer | image | subject | label | |
| OpenGTA | Tok | ? | Linux | action/reflex racing/driving | | | label | |
| OpenQuartz | Open Quartz Team | 2001 | Linux | shooter | | subject | label | |
| OpenQuartz | Open Quartz Team | ? | BeOS | shooter | | | label | |
| OpenSpades | author | 2013 | Linux | shooter | | | label | |
| Operation C | Konami | 1991 | GB | platformer shooter Science Fiction | image | subject | label | |
| Operation Flashpoint: Dragon Rising | Codemasters | 2009 | Windows | shooter | image | subject | label | |
| Operation Flashpoint: Dragon Rising | Codemasters | 2009 | PS3 | shooter | image | subject | label | |
| Operation Flashpoint: Dragon Rising | Codemasters | 2009 | X360 | shooter | image | subject | label | |
| Operation Flashpoint: Red River | Codemasters Action Studio | 2011 | Windows | shooter | | subject | label | |
| Operation Flashpoint: Red River | Codemasters Action Studio | 2011 | PS3 | shooter | | | label | |
| Operation G.H.O.S.T. | Sega | 2012 | Arcade | shooter | image | | label | |
| Operation Thunderbolt | Ocean | 1989 | ZX Spectrum | shooting gallery | image | | label | |
| Operation Thunderbolt | Ocean | 1990 | C64 | shooting gallery | image | | label | |
| Operation Thunderbolt | Ocean | 1989 | Atari ST | shooting gallery | image | | label | |
| Operation Thunderbolt | Ocean | 1989 | Amstrad CPC | shooting gallery | image | | label | |
| Operation Wolf | Taito | 1990 | Master System | shooting gallery | image | subject | label | |
| Operation Wolf | Ocean;Taito | 1988 | Amiga | shooter | image | | label | |
| Operation Wolf | Ocean | 1988 | Atari ST | shooting gallery | image | | label | |
| Operation Wolf | Taito | 1989 | MS-DOS | shooting gallery | image | | label | |
| Operation Wolf | Ocean | 1988 | MSX | shooting gallery | image | | label | |
| Operation Wolf | Ving | 1990 | FM Towns | shooting gallery | image | | label | |
| Operation Wolf | Toybox | 2006 | MSX | shooting gallery | image | | label | |
| Operation Wolf | Ocean | 1988 | ZX Spectrum | shooting gallery | image | | label | |
| Operation Wolf | Ocean | 1988 | C64 | shooting gallery | image | | label | |
| Operation Wolf | Ocean | 1988 | Amstrad CPC | shooting gallery | image | | label | |
| Operation Wolf | NEC Avenue | 1990 | PC Engine | shooting gallery | image | | label | |
| Operation Wolf | Taito | 1989 | NES | shooting gallery | image | subject | label | |
| Operation Wolf | Taito | 1987 | Arcade | shooting gallery | image | | label | |
| Operation: Matriarchy | MADia Entertainment | 2005 | Windows | shooter Science Fiction | image | subject | label | |
| Operation: Pedopriest | Molleindustria | 2005 | Linux | platformer | image | | label | |
| Operation: Pedopriest | Molleindustria | 2005 | Flash | platformer | | | label | |
| OQPlus | author | ? | Linux | shooter | | | label | |
| OQPlus | ? | ? | BeOS | shooter | | | label | |
| Organ Trail: Complete Edition | The Men Who Wear Many Hats | 2015 | PS Vita | shooter | image | | label | |
| Organ Trail: Director's Cut | Hats Productions | 2012 | Linux | shooter | image | | label | |
| Out of Hell | author | 2008 | Linux | Horror | | | label | |
| Outcast | Appeal;Fresh3D | 1999 | Windows | action/reflex adventure Science Fiction | image | subject | label | |
| Outcast Reboot HD | Freash3D | 2015 | Linux | action/reflex adventure Science Fiction | | | label | |
| Outlaws | LucasArts | 1997 | Windows | shooter | | | label | |
| Outpost:Haven | Garage Collective | 2011 | Flash | shooter Science Fiction | | | label | |
| Outpost:Swarm | Garage Collective | 2012 | Flash | shooter Science Fiction | | | label | |
| Outwars | SingleTrac | 1998 | Windows | shooter Science Fiction | image | subject | label | |
| OverDose | Team Blur Games | ? | Linux | shooter | | | label | |
| Overland | Finji | 2016 | Linux | strategy | | | label | |
| Overwatch | Blizzard Entertainment | 2016 | Windows | action/reflex shooter Science Fiction | | | label | |
