| onx | ONYX PosterShop Packaged Job |
| ooa | OOA Interchange Format Data |
| oob | Sybase PowerDesigner Object-oriented Model Backup |
| ooc | CHARTrunner Out-of-control Tests Definition |
| oodbri | O&O Defrag Binary Report Data |
| ook | Arachne Batch Script |
| ool | Orbit GIS Object List |
| oom | Sybase PowerDesigner Object-oriented Model |
| ooxml | Office Open XML File |
| op2 | Nastran Binary Results |
| op4 | Nastran Binary Results |
| opal | Microsoft Office Language Specific Resources |
| 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 |