| odd | TEI Source Code |
| odex | Android Optimized Dalvik Executable |
| odg | Psion OVAL Dialog File |
| odl | Object Description Language |
| odp | OPRO Direct to Print File |
| oem | Windows OEM Setup File |
| ofd | OPRO Form Data |
| ofp | Objecteering Project |
| ofr | Psion OVAL Form |
| ogd | Oracle Forms Graph |
| ogre | OGRE 3D Data |
| ohc | Delphi OpenHelp Contents |
| ohi | Delphi OpenHelp Index |
| ohl | Delphi OpenHelp Links |
| oks | OKScript Script |
| okx | OKScript Script |
| olb | Oracle Forms Object Library Module Binary |
| olb | Synergy/DE Object Library |
| ole | Object Linking and Embedding Data |
| oln | Microsoft Visual C++ Outline Examples Data |
| olt | Oracle Forms Object Library Module Text |
| om | OMake Makefile |
| omc | OMake Cache File |
| omh | Omnis Studio Header |
| omk | Ocera Make System Script |
| omo | OMake Object File |
| oo | FreePascal Object |
| oot | Python pytest-oot Test |
| op | Origin Preprocessed Data |
| opa | OPA Source Code |
| opentag | OpenTag Document |
| opf | Obsidium Project File |
| opo | Psion OPL Object |
| oppo | Oppo Core |
| opr | OPRO Print Report |