| onebin | Microsoft OneNote Binary Data |
| onecache | Microsoft OneNote Cached Data |
| oneffects | Perfect Effects Effects Data |
| onepkg | Microsoft OneNote Package |
| onetmp | Microsoft OneNote Temporary File |
| onetoc | Microsoft OneNote Table of Contents |
| onetoc2 | Microsoft OneNote Table of Contents |
| onl | PATCHMASTER Online Analysis Information |
| onm | Outdoor Navigator Map |
| ont | theWord Bible Text Module |
| ontx | theWord Encrypted Bible Text Module |
| onw | Omron NTST Data |
| onx | ONYX PosterShop Packaged Job |
| ooa | OOA Interchange Format Data |
| oob | Sybase PowerDesigner Object-oriented Model Backup |
| ooc | CHARTrunner Out-of-control Tests Definition |
| 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 |
| 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 |
| 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 |