| opal | Microsoft Office Language Specific Resources |
| opb | Bryce Object Data |
| opb | Gurobi Optimizer Pseudo-boolean Optimization Data |
| opc | Office Upgrade Control |
| opc | Orbit Point Cloud Data |
| opcses | Opticodec-PC Session Data |
| opd | Wyko NT9100 Surface Profiler Surface Data |
| opdownload | Opera Partially Downloaded File |
| ope | IRCS Observation Procedure Execution Data |
| openbus | Altium Designer OpenBus System Document |
| opf | Concordance Print Settings 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 | Psion Organiser OPL Source |
| options | Eclipse IDE Trace Options |
