| | Offroad Thunder | 2000 | Midway Games | offrthnd | | 0.00 |
| | Oh La La (Stella) | 1969 | Stella | st_ohla | | 0.00 |
 | Oh My God! (Japan) | 1993 | Atlus | ohmygod | Puzzle / Drop | 9.00 |
| | Oh! Paipee (Japan 890227) | 1989 | Nichibutsu | ohpaipee | | 0.00 |
 | Ohgon no Siro (Japan) (clone of: gladiatr) | 1986 | Taito Corporation | ogonsiro | Fighter / 2D | 0.00 |
| | Oigas (bootleg) (clone of: gigas) | 1986 | bootleg | oigas | Breakout | 0.00 |
| | Oinori-daimyoujin Matsuri | 2003 | Sega | oinori | | 0.00 |
 | Oishii Puzzle Ha Irimasenka | 1993 | Sunsoft / Atlus | oisipuzl | Mini-Games | 8.00 |
 | Ojanko Club (Japan) | 1986 | V-System Co. | ojankoc | Tabletop / Mahjong *Mature* | 0.00 |
 | Ojanko High School (Japan) | 1988 | V-System Co. | ojankohs | Tabletop / Mahjong *Mature* | 4.00 |
 | Ojanko Yakata (Japan) | 1986 | V-System Co. | ojankoy | Tabletop / Mahjong *Mature* | 3.00 |
 | Ojanko Yakata 2bankan (Japan) | 1987 | V-System Co. | ojanko2 | Tabletop / Mahjong *Mature* | 0.00 |
 | Ojousan (Japan 871204) | 1987 | Nichibutsu | ojousan | Tabletop / Mahjong *Mature* | 2.00 |
| Ojousan [BET] (Japan 870108) (clone of: ojousan) | 1987 | Nichibutsu | ojousanm | Tabletop / Mahjong *Mature* | 0.00 |
| | Old Coney Island! | 1979 | Game Plan | coneyis | | 0.00 |
| | Old Timer (Barcrest) (Dutch) (MPU4) (DOT1.1) | 1969 | Barcrest | m4oldtmr | | 10.00 |
| | Old Timer (Barcrest) (Dutch, alt 'Black and White' sound roms) (DOT1.1) (clone of: m4oldtmr) | 1969 | Barcrest | m4casot | | 0.00 |
| | Old Timer (Barcrest) (Dutch, alt 'JPM Classic' sound roms) (DOT1.1) (clone of: m4oldtmr) | 1969 | Barcrest | m4jpmcla | | 0.00 |
 | Oli-Boo-Chu | 1981 | Irem / GDI | olibochu | Maze | 4.00 |
| | Ollie King (GDX-0007) | 2004 | Sega | ollie | | 0.00 |
