| MAME Name | Description | Official | Alternate | VC WS | ||
 | offroad | Ironman Ivan Stewart's Super Off-Road (rev 4) | 680 kB 20-Jan-2019 Download | - | 272 kB 07-May-2021 Download | |
 | offroadc | Off Road Challenge (v1.63) | - | - | 1,10 MB 05-May-2021 Download | |
| offroadt | Ironman Ivan Stewart's Super Off-Road Track-Pak | - | - | 285 kB 07-May-2021 Download | |
 | offtwall | Off the Wall (2/3-player upright) | 2,81 MB 20-Nov-2015 Download | - | 316 kB 02-Apr-2021 Download | |
| ohmygod | Oh My God! (Japan) | - | - | 169 kB 02-Apr-2021 Download | |
| oisipuzl | Oishii Puzzle Ha Irimasenka | - | - | 198 kB 02-Apr-2021 Download | |
| olibochu | Oli-Boo-Chu | - | - | 149 kB 02-Apr-2021 Download | |
| omegaf | Omega Fighter | - | - | 103 kB 02-Apr-2021 Download | |
 | omegrace | Omega Race (set 1) | 13,4 MB 03-Jul-2017 Download | - | 14,0 MB 04-May-2021 Download | |
 | one4all | One For All (0101503V, New Zealand) | - | 625 kB 16-Feb-2019 Download | - | |
| oneshot | One Shot One Kill | - | - | 248 kB 05-May-2021 Download | |
| onna34ro | Onna Sanshirou - Typhoon Gal | - | - | 58,2 kB 02-Apr-2021 Download | |
| opaopa | Opa Opa (MC-8123, 317-0042) | - | - | 194 kB 02-Apr-2021 Download | |
 | openice | 2 On 2 Open Ice Challenge (rev 1.21) | - | - | 1,50 MB 07-May-2021 Download | |
 | opwolf | Operation Wolf (World, set 1) | 3,50 MB 20-Nov-2015 Download | - | 5,56 MB 07-May-2021 Download | |
 | opwolf3 | Operation Wolf 3 (World) | 291 kB 20-Nov-2015 Download | - | 311 kB 02-Apr-2021 Download | |
 | orbit | Orbit | 529 kB 20-Nov-2015 Download | - | 209 kB 02-Apr-2021 Download | |
| orbitron | Orbitron | - | - | 485 kB 02-Apr-2021 Download | |
| orbs | Orbs (10/7/94 prototype?) | - | - | 238 kB 02-Apr-2021 Download | |
 | orchidms | Orchid Mist (0200849V, NSW/ACT) | - | 630 kB 16-Mar-2019 Download | - | |
 | orchidmsnz | Orchid Mist (0101241V, New Zealand) | - | 629 kB 16-Mar-2019 Download | - | |
| ordyne | Ordyne (World) | - | - | 263 kB 02-Apr-2021 Download | |
| orlegend | Oriental Legend / Xiyou Shi E Chuan (ver. 126) | - | - | 159 kB 02-Apr-2021 Download | |
| orunners | OutRunners (World) | - | - | 955 kB 07-May-2021 Download | |
 | osbexec | Executive | 11,8 MB 03-Jun-2018 Download | - | - | |
 | osborne1 | Osborne-1 | 4,07 MB 03-Jun-2018 Download | - | - | |
| oscar | Psycho-Nics Oscar (World revision 0) | - | - | 188 kB 02-Apr-2021 Download | |
 | oscara5 | Oscar (0200348V, NSW/ACT) | - | 629 kB 16-Feb-2019 Download | - | |
 | osman | Osman (World) | 1,74 MB 20-Nov-2015 Download | - | 4,01 MB 04-May-2021 Download | |
| othldrby | Othello Derby (Japan) | - | - | 223 kB 02-Apr-2021 Download | |
 | othunder | Operation Thunderbolt (World, rev 1) | 263 kB 20-Nov-2015 Download | - | 66,9 kB 02-Apr-2021 Download | |
 | otwalls | Off the Wall (Sente) | 178 kB 20-Nov-2015 Download | - | - | |
 | outfxies | The Outfoxies (World, OU2) | 1,79 MB 20-Nov-2015 Download | - | 195 kB 02-Apr-2021 Download | |
 | outrun | Out Run (sitdown/upright, Rev B) | 20,1 MB 18-Aug-2019 Download | - | 2,17 MB 04-May-2021 Download | |
 | outzone | Out Zone | 25,1 MB 20-Jan-2019 Download | - | 1,85 MB 07-May-2021 Download | |
 | overtop | Over Top | - | - | 2,30 MB 04-May-2021 Download | |
 | oz750 | Wizard OZ-750 | 1,21 MB 16-Mar-2019 Download | - | - | |
| ozmawars | Ozma Wars (set 1) | - | - | 117 kB 02-Apr-2021 Download | |
| ozon1 | Ozon I | - | - | 423 kB 02-Apr-2021 Download | |
