| nx3 | Rappelz NX3 Data |
| o3d | OMSI 3D Data |
| oa0 | B.O.U.T. Evolution Texture Data |
| oac | Grand Theft Auto V Audio Data |
| oav | Microsoft Flight Simulator Texture Data |
| obg | OpenTTD OpenGFX Base Graphics Data |
| odr | OpenIV Model Data |
| ogf | Stalker Model Data |
| ogz | Cube 2 Map Data |
| oiv | OpenIV Mod Package File |
| ojm | O2Jam Music File |
| ojn | O2Jam Note File |
| ojs | O2Jam Image File |
| ojt | O2Jam Image File |
| ojt | O2Jam Music File |
| ojt | O2Jam Note File |
| ol | Valve Hammer Object Layout |
| omod | Oblivion Mod Archive |
| omw | Omweso Game Data |
| oncc | Oni Game Character Data |
| oni | Oni Game Data |
| onlv | Oni Game Level Data |
| oob | HPS Ancient Warfare Army Data |
| oof | Descent 3 Outrage 3D Model |
| oppc | Darksiders Game Package |
| opps3 | Darksiders Game Package |
| ork | Warhammer: Mark of Chaos Game Archive |
| orz | Holic Online Game Data |
| osb | Osu! Beatmap Script |
| osf | Descent 3 Outrage Sound |
| osk | Osu! Skin File |
| osn | OMSI Saved Situation |
| osr | Osu! Replay File |
| osu | Osu! Script |
| otd | OpenIV Texture Data |