| olb | Synergy/DE Object Library |
| olb | Oracle Forms Object Library Module Binary |
| 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 |
| opt | Microsoft Visual Studio Workspace Options |
| opt | ProWorx NXT List Module Option File |
| orderedtest | Visual Studio Ordered Test Data |
| ors | Psion OVAL Resource File |
| os | ObjectScript Source Code |
| osax | AppleScript Scripting Addition |
| osd | Open Software Description File |
| osl | Open Shading Language Source |
| oso | Open Shading Compiled Object |
| otf | VAMPIR Open Trace Format Data |
| otf2 | VAMPIR Open Trace Format Data |
| ovd | Borland ObjectVision Source |
| ovd | NetCOBOL PowerFORM Overlay Descriptor |
| ovg | ObjectVision Converted Graphic Data |
| ovp | Psion OVAL Project |
| ovx | Psion OVAL Control |