| Game Name | System |
|---|
| Ondra | MAME |
| Ondra ViLi | MAME |
| One + Two | MAME |
| One + Two (earlier) | MAME |
| One Shot One Kill | MAME |
| Oni - The Ninja Master (Japan) | MAME |
| Only Fools & Horses (Bellfruit) (set 1) (Scorpion 2/3) | MAME |
| Only Fools & Horses (Bellfruit) (set 2) (Scorpion 2/3) | MAME |
| Only Fools & Horses (Bellfruit) (set 3) (Scorpion 2/3) | MAME |
| Only Fools & Horses (Bellfruit) (set 4) (Scorpion 2/3) | MAME |
| Only Fools & Horses (Bellfruit) (set 5) (Scorpion 2/3) | MAME |
| Only Fools and Horses (Bellfruit) (Scorpion 1?) | MAME |
| Onna Sansirou - Typhoon Gal (set 1) | MAME |
| Onna Sansirou - Typhoon Gal (set 2) | MAME |
| Onyx (Brazil/Prototype) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.0 B) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.0 D) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.0 K) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.0, set 1) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.0, set 2) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.1 B) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.1 D) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.1 K) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU4) (DR_ 2.1) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU5) (set 1) | MAME |
| Ooh Aah Dracula (Barcrest) (MPU5) (set 2) | MAME |
| Ooh Aah Dracula Club (Barcrest) (MPU5) (set 1) | MAME |
| Ooh Aah Dracula Club (Barcrest) (MPU5) (set 2) | MAME |
| Ooh Aah Dracula Club (Barcrest) (MPU5) (set 3) | MAME |
| Ooh Ahh Dracula Rio (Barcrest) (MPU5) | MAME |
| Oozumou - The Grand Sumo (DECO Cassette) (Japan) | MAME |
| Opa Opa (MC-8123, 317-0042) | MAME |
| Open Mahjong [BET] (Japan) | MAME |
| Open Sesame (0351309, US) | MAME |
| Open Sesame (Bellfruit) (Scorpion 4) (set 1) | MAME |
| Open Sesame (Bellfruit) (Scorpion 4) (set 2) | MAME |
| Open The Box (Ace) (sp.ACE) (set 1) | MAME |
| Open The Box (Ace) (sp.ACE) (set 10) | MAME |
| Open The Box (Ace) (sp.ACE) (set 2) | MAME |
| Open The Box (Ace) (sp.ACE) (set 3) | MAME |
| Open The Box (Ace) (sp.ACE) (set 4) | MAME |
| Open The Box (Ace) (sp.ACE) (set 5) | MAME |
| Open The Box (Ace) (sp.ACE) (set 6) | MAME |
| Open The Box (Ace) (sp.ACE) (set 7) | MAME |
| Open The Box (Ace) (sp.ACE) (set 8) | MAME |
| Open The Box (Ace) (sp.ACE) (set 9) | MAME |
| Open The Box (Barcrest) (MPU5) (set 1) | MAME |
| Open The Box (Barcrest) (MPU5) (set 2) | MAME |
| Open The Box (Barcrest) (MPU5) (set 3) | MAME |
| Open The Box (Barcrest) (MPU5) (set 4) | MAME |
| Operation Bear (bootleg of Operation Wolf) | MAME |
| Operation Thunder Hurricane (ver EAA) | MAME |
| Operation Thunder Hurricane (ver UAA) | MAME |
| Operation Thunderbolt (Japan) | MAME |
| Operation Thunderbolt (US) | MAME |
| Operation Thunderbolt (US, rev 1) | MAME |
| Operation Thunderbolt (World) | MAME |
| Operation Thunderbolt (World, rev 1) | MAME |
| Operation Tiger | MAME |
| Operation Wolf (Japan) | MAME |
| Operation Wolf (Japan, prototype) | MAME |
| Operation Wolf (US) | MAME |
| Operation Wolf (World, set 1) | MAME |
| Operation Wolf (World, set 2) | MAME |
| Operation Wolf 3 (US) | MAME |
| Operation Wolf 3 (World) | MAME |
| Operation: Thunder | MAME |
| Option 3 (v1.0) (Datapak) (MPU4 Video) | MAME |
| Option 3 (v1.0) (MPU4 Video) | MAME |
| Options Unlimited (Barcrest) (MPU3) | MAME |
| Oracle (Dutch) (Bellfruit) (Scorpion 1) | MAME |
| Orange Club - Maru-hi Ippatsu Kaihou [BET] (Japan 880221) | MAME |
| Orange Club - Maruhi Kagai Jugyou (Japan 880213) | MAME |
| Orao 102 | MAME |
| Orao 103 | MAME |
| Orbatak (prototype) | MAME |
| Orbit | MAME |
| Orbit 1 | MAME |
| Orbitor 1 | MAME |
| Orbitron | MAME |
| Orbs (10/7/94 prototype?) | MAME |
| Orchid Mist (0100849V, NSW/ACT) | MAME |
| Orchid Mist (0200849V, NSW/ACT) | MAME |
| Ordisavant (France) | MAME |
| Ordyne (Japan) | MAME |
| Ordyne (Japan, English Version) | MAME |
| Organiser I | MAME |
| Organiser II CM | MAME |
| Organiser II LA | MAME |
| Organiser II LAM | MAME |
| Organiser II LZ | MAME |
| Organiser II LZ64 | MAME |
| Organiser II LZ64S | MAME |
| Organiser II P200 | MAME |
| Organiser II P350 | MAME |
| Organiser II P464 | MAME |
| Organiser II XP | MAME |
| Oric 1 | MAME |
| Oric Atmos | MAME |
| Oric Telestrat | MAME |
