| 199? | | Oni - The Ninja Master (Japan) | Banpresto / Pandorabox | 1995 |
| Onna Sansirou - Typhoon Gal (set 1) | Taito | 1985 |
| Onna Sansirou - Typhoon Gal (set 2) | Taito | 1985 |
| Opa Opa (MC-8123, 317-0042) | Sega | 1987 |
| Open Mahjong [BET] (Japan) | Sapporo Mechanic | 1981? |
| Operation Bear (bootleg of Operation Wolf) | bootleg (Bear Corporation Korea) | 1987 |
| Operation Thunder Hurricane (ver EAA) | Konami | 1996 |
| Operation Thunder Hurricane (ver UAA) | Konami | 1996 |
| Operation Thunderbolt (Japan) | Taito Corporation | 1988 |
| Operation Thunderbolt (US) | Taito America Corporation | 1988 |
| Operation Thunderbolt (US, older) | Taito America Corporation | 1988 |
| Operation Thunderbolt (World) | Taito Corporation Japan | 1988 |
| Operation Tiger | Taito | 1998 |
| Operation Wolf (Japan) | Taito Corporation | 1987 |
| Operation Wolf (US) | Taito America Corporation | 1987 |
| Operation Wolf (World, set 1) | Taito Corporation Japan | 1987 |
| Operation Wolf (World, set 2) | Taito Corporation Japan | 1987 |
| Operation Wolf 3 (US) | Taito America Corporation | 1994 |
| Operation Wolf 3 (World) | Taito Corporation Japan | 1994 |
| Orange Club - Maru-hi Ippatsu Kaihou [BET] (Japan 880221) | Daiichi Denshi | 1988 |
| Orange Club - Maruhi Kagai Jugyou (Japan 880213) | Daiichi Denshi | 1988 |
| Orbit | Atari | 1978 |
| Orbitron | Signatron USA | 1982 |
| Orbs (10/7/94 prototype?) | American Sammy | 1994 |
| Ordyne (Japan) | Namco | 1988 |
| Ordyne (Japan, English Version) | Namco | 1988 |
| Oriental Legend / Xi You Shi E Zhuan (ver. 105, Korean Board) | IGS | 1997 |
| Oriental Legend / Xi You Shi E Zhuan (ver. 111, Chinese Board) | IGS | 1997 |
| Oriental Legend / Xi You Shi E Zhuan (ver. 112) | IGS | 1997 |
| Oriental Legend / Xi You Shi E Zhuan (ver. 112, Chinese Board) | IGS | 1997 |
| Oriental Legend / Xi You Shi E Zhuan (ver. 126) | IGS | 1997 |
| Oriental Legend / Xi You Shi E Zhuan (ver. ???, Chinese Board) | IGS | 1997 |
| Oriental Legend Special / Xi You Shi E Zhuan Super (alt ver. 100) | IGS | 1998 |
| Oriental Legend Special / Xi You Shi E Zhuan Super (ver. 100) | IGS | 1998 |
| Oriental Legend Special / Xi You Shi E Zhuan Super (ver. 101, Korean Board) | IGS | 1998 |
| Oriental Legend Special Plus / Xi You Shi E Zhuan Super Plus | IGS | 2004 |
| Osman (World) | Mitchell | 1996 |
| Otenami Haiken (V2.04J) | Success | 1999 |
| Otenami Haiken Final (V2.07JC) | Success / Warashi | 2005 |
| Otenki Kororin (V2.01J) | Takumi | 2001 |
| Othello (version 3.0) | Success | 1984 |
| Othello Derby (Japan) | Sunwise | 1995 |
| Othello Shiyouyo (J 980423 V1.002) | Success | 1998 |
| Otogizoushi Urashima Mahjong (Japan) | UPL | 1989 |
| Otona no Mahjong (Japan 880628) | Apple | 1988 |
| Out Run (bootleg) | bootleg | 1986 |
| Out Run (deluxe sitdown) | Sega | 1986 |
| Out Run (Mega-Tech, SMS based) | Sega | 1987 |
| Out Run (sitdown/upright) | Sega | 1986 |
| Out Run (sitdown/upright, Rev A) | Sega | 1986 |
| Out Run (sitdown/upright, Rev B) | Sega | 1986 |
| Out Run 2 (Rev. A) (GDX-0004A) | Sega | 2003 |
| Out Zone (set 1) | Toaplan | 1990 |
| Out Zone (set 2) | Toaplan | 1990 |
| Out Zone (set 3, prototype?) | Toaplan | 1990 |
| Out Zone (set 4) | Toaplan | 1990 |
| Out Zone (set 5) | Toaplan | 1990 |
| Outfoxies (Japan, OU1) | Namco | 1994 |
| Outfoxies (World, OU2) | Namco | 1994 |
| Outline | Century Electronics | 1982 |
| OutRunners (US) | Sega | 1992 |
| OutRunners (World) | Sega | 1992 |
| OutTrigger (JPN, USA, EXP, KOR, AUS) | Sega | 1999 |
| Over Drive | Konami | 1990 |
| Over Rev (Revision A) | Jaleco | 1997 |
| Over Top | ADK | 1996 |
| Ozma Wars (set 1) | SNK | 1979 |
| Ozma Wars (set 2) | SNK | 1979 |
| Ozon I | Proma | 1983 |