| Game Name | System |
|---|
| Oh! Paipee (Japan 890227) | MAME |
| Oigas (bootleg) | MAME |
| Oinori-daimyoujin Matsuri | MAME |
| Oishii Puzzle Ha Irimasenka | MAME |
| Ojanko Club (Japan) | MAME |
| Ojanko High School (Japan) | MAME |
| Ojanko Yakata (Japan) | MAME |
| Ojanko Yakata 2bankan (Japan) | MAME |
| Ojousan (Japan 871204) | MAME |
| Ojousan [BET] (Japan 870108) | MAME |
| Okeah-240 | MAME |
| Old Coney Island! | MAME |
| Old Timer (Barcrest) (Dutch) (MPU4) (DOT1.1) | MAME |
| Old Timer (Barcrest) (Dutch, alt 'Black and White' sound roms) (DOT1.1) | MAME |
| Old Timer (Barcrest) (Dutch, alt 'JPM Classic' sound roms) (DOT1.1) | MAME |
| Oli-Boo-Chu | MAME |
| Olivetti L1 M20 | MAME |
| Olivetti L1 M40 | MAME |
| Ollie King (GDX-0007) | MAME |
| Olympia (Dutch) (Bellfruit) (Scorpion 1) | MAME |
| Olympian Games (Russia) | MAME |
| Olympic Games | MAME |
| Olympic Gold (Bellfruit) (set 1, UK) (Scorpion 2/3) | MAME |
| Olympic Gold (Bellfruit) (set 1, UK, Protocol) (Scorpion 2/3) | MAME |
| Olympic Gold (Bellfruit) (set 2, UK, 10GBP Jackpot) (Scorpion 2/3) | MAME |
| Olympic Gold (Bellfruit) (set 2, UK, 10GBP Jackpot, Protocol) (Scorpion 2/3) | MAME |
| Olympic Gold (German) (Nova) (MPU4) (set 1) | MAME |
| Olympic Gold (German) (Nova) (MPU4) (set 2) | MAME |
| Olympic Hot Stuff (TAS 5 Reel System) | MAME |
| Olympic Soccer '92 (set 1) | MAME |
| Olympic Soccer '92 (set 2) | MAME |
| Olympic Soccer '92 (set 3) | MAME |
| Olympus | MAME |
| Omega | MAME |
| Omega (Barcrest) (Dutch) (MPU4) | MAME |
| Omega (Maygay) (M1A/B) (set 1) | MAME |
| Omega (Maygay) (M1A/B) (set 2) | MAME |
| Omega (bootleg?) | MAME |
| Omega Fighter | MAME |
| Omega Fighter Special | MAME |
| Omega Race (set 1) | MAME |
| Omega Race (set 2) | MAME |
| Omni | MAME |
| Omni (Shuffle) (L-1) | MAME |
| Omni II Logic Analyzer | MAME |
| Omotesandou (Japan 890215) | MAME |
| On Line (Pcp) (MPU3) | MAME |
| On The Box (Ace) (sp.ACE) (set 1) | MAME |
| On The Box (Ace) (sp.ACE) (set 10) | MAME |
| On The Box (Ace) (sp.ACE) (set 11) | MAME |
| On The Box (Ace) (sp.ACE) (set 12) | MAME |
| On The Box (Ace) (sp.ACE) (set 13) | MAME |
| On The Box (Ace) (sp.ACE) (set 14) | MAME |
| On The Box (Ace) (sp.ACE) (set 15) | MAME |
| On The Box (Ace) (sp.ACE) (set 2) | MAME |
| On The Box (Ace) (sp.ACE) (set 3) | MAME |
| On The Box (Ace) (sp.ACE) (set 4) | MAME |
| On The Box (Ace) (sp.ACE) (set 5) | MAME |
| On The Box (Ace) (sp.ACE) (set 6) | MAME |
| On The Box (Ace) (sp.ACE) (set 7) | MAME |
| On The Box (Ace) (sp.ACE) (set 8) | MAME |
| On The Box (Ace) (sp.ACE) (set 9) | MAME |
| On The Buses (Maygay) (M1A/B) (set 1) | MAME |
| On The Buses (Maygay) (M1A/B) (set 10) | MAME |
| On The Buses (Maygay) (M1A/B) (set 11) | MAME |
| On The Buses (Maygay) (M1A/B) (set 12) | MAME |
| On The Buses (Maygay) (M1A/B) (set 13) | MAME |
| On The Buses (Maygay) (M1A/B) (set 14) | MAME |
| On The Buses (Maygay) (M1A/B) (set 15) | MAME |
| On The Buses (Maygay) (M1A/B) (set 16) | MAME |
| On The Buses (Maygay) (M1A/B) (set 17) | MAME |
| On The Buses (Maygay) (M1A/B) (set 2) | MAME |
| On The Buses (Maygay) (M1A/B) (set 3) | MAME |
| On The Buses (Maygay) (M1A/B) (set 4) | MAME |
| On The Buses (Maygay) (M1A/B) (set 5) | MAME |
| On The Buses (Maygay) (M1A/B) (set 6) | MAME |
| On The Buses (Maygay) (M1A/B) (set 7) | MAME |
| On The Buses (Maygay) (M1A/B) (set 8) | MAME |
| On The Buses (Maygay) (M1A/B) (set 9) | MAME |
| On The Fiddle (Barcrest) (MPU5) (set 1) | MAME |
| On The Fiddle (Barcrest) (MPU5) (set 2) | MAME |
| On The Piste (Ace) (sp.ACE) (set 1) | MAME |
| On The Piste (Ace) (sp.ACE) (set 10) | MAME |
| On The Piste (Ace) (sp.ACE) (set 11) | MAME |
| On The Piste (Ace) (sp.ACE) (set 12) | MAME |
| On The Piste (Ace) (sp.ACE) (set 13) | MAME |
| On The Piste (Ace) (sp.ACE) (set 14) | MAME |
| On The Piste (Ace) (sp.ACE) (set 15) | MAME |
| On The Piste (Ace) (sp.ACE) (set 16) | MAME |
| On The Piste (Ace) (sp.ACE) (set 17) | MAME |
| On The Piste (Ace) (sp.ACE) (set 2) | MAME |
| On The Piste (Ace) (sp.ACE) (set 3) | MAME |
| On The Piste (Ace) (sp.ACE) (set 4) | MAME |
| On The Piste (Ace) (sp.ACE) (set 5) | MAME |
| On The Piste (Ace) (sp.ACE) (set 6) | MAME |
| On The Piste (Ace) (sp.ACE) (set 7) | MAME |
| On The Piste (Ace) (sp.ACE) (set 8) | MAME |
| On The Piste (Ace) (sp.ACE) (set 9) | MAME |
| On The Up (Mazooma) (Scorpion 4) (set 1) | MAME |
| On The Up (Mazooma) (Scorpion 4) (set 2) | MAME |
