| opdownload | Opera Partially Downloaded File |
| ope | IRCS Observation Procedure Execution Data |
| openbus | Altium Designer OpenBus System Document |
| opf | Concordance Print Settings Data |
| opf | FlipAlbum FlipBook Data |
| opi | Adlib Express OCR Processing Information |
| opi | OpenPanorama Image File |
| opj | OmniPage Job Data |
| opl | OpenPanorama Lensflare File |
| opm | Quadrox Digital CCTV System Components Data |
| opop | SOCSIM Output Population File |
| opox | SOCSIM Output Extra Variables File |
| opp | OpenPanorama Panorama File |
| opq | Eschalon: Book I Game Data |
| oprc | Palm OS Desktop OPRC Data |
| ops | Altair FEKO OPTFEKO Internal Data |
| ops | Dynamite VSP Object Placement Style Data |
| ops | Orbit Point Cloud Selection Data |
| opt | Altair FEKO CADFEKO Optimisation Data |
| opt | Concordance Native Viewer Database |
| opt | CRiSP Harvest Output Path Data |
| opt | CX-Programmer OPT Data |
| opt | Digital Line Graph Optional Vector Data |
| opt | Psion Organiser OPL Source |
| options | Eclipse IDE Trace Options |
| opuscert | Directory Opus Certificate Data |
| opv | OpenPanorama Visit File |
| opw | eSignal Options Data |
| opx | FlipAlbum PhotoAlbum Data |
| opx | Psion Organiser OPX Library |
| opy | OptiY Model |
| op_ | GKSetup Support Data |
| or2 | Lotus Organizer 2 Data |
| or3 | Lotus Organizer 97 Data |
| or4 | Lotus Organizer 4 Data |
