| Othello 2000 | Othello Millennium | Tsukada Original | 08 October 1999 | Boardgame | GB | N | N | Y | N |
| Otenba Becky no Daibouken | Tomboyish Becky's Large Adventure | MIA | 1983 | Action > Platformer | MSX | N | N | Y | N |
| Otaku no Seiza: Adventure in the Otaku Galaxy | The Constellation Nerdulus - An Adventure in the Otaku Galaxy | M&M | 31 July 1991 | Role Playing | NES | N | N | Y | N |
| Oriental Blue: Ao no Tengai | No Alternate Title | Nintendo | 24 October 2003 | Role Playing | GBA | N | N | Y | N |
| Orguss | No Alternate Title | Sega | 1984 | Action > Shooter | SG1K | N | N | Y | N |
| Ordyne | No Alternate Title | NEC | 08 September 1989 | Action > Shooter | TG16 | N | N | Y | N |
| Opoona | No Alternate Title | Koei | 01 November 2007 | Role Playing | WII | N | N | N | Y |
| Operation: Secret Storm | No Alternate Title | Color Dreams | 1992 | Action | NES | N | N | N | Y |
| Operation Wolf | No Alternate Title | Taito | 31 March 1989 | Action > Shooter | NES | N | N | Y | N |
| Operation Wolf | No Alternate Title | Sega | 1988 | Action > Shooter | SMS | N | N | Y | N |
| Operation C | Contra;Probotector | Konami | 08 January 1991 | Action > Platformer | GB | N | N | Y | Y |
| Onyanko Town | Dog Town | Pony Canyon | 21 November 1985 | Action | NES | N | N | Y | N |
| Onslaught | No Alternate Title | Ballistic | 1991 | Action > Platformer | GEN | N | N | Y | N |
| Oni IV: Kishin no Ketsukozoku | No Alternate Title | Banpresto | 11 March 1994 | Role Playing | GB | N | N | Y | N |
| On the Ball | Cameltry | Taito | 26 June 1992 | Action | SNES | N | N | N | Y |
| Okamoto Ayako to Match Play Golf: Ko Olina Golf Club in Hawaii | Ayako Okamoto and the Match Play Golf: Ko Olina Golf Club in Hawaii | Tsukuda Original | 21 September 1994 | Sports | SNES | N | N | Y | N |
| Okami HD | No Alternate Title | Capcom | 12 December 2017 | Action Adventure | PC | N | Y | N | N |
| Okami | Ookami | Capcom | 20 April 2006 | Role Playing > Action RPG | PS2 | N | N | Y | N |
| Okaeri! Chibi-Robo! Happy Richie Oosouji | Chibi-Robo: Clean Sweep! | Nintendo | 23 July 2009 | Action Adventure | NDS | N | N | Y | N |
| Ojou-sama no DokiDoki Daisakusen: Nowareta Ko no Kan | The Mistress’ Heart-Throbbing Adventure;Remidoki | Blue-Mica | 14 August 2010 | Action > Platformer | PC | N | N | Y | N |
