| opt | Microsoft Visual Studio Workspace Options |
| 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 |
| ovd | Borland ObjectVision Source |
| ovd | NetCOBOL PowerFORM Overlay Descriptor |
| ovg | ObjectVision Converted Graphic Data |
| ovp | Psion OVAL Project |
| ovx | Psion OVAL Control |
| ow | FreePascal 1.0.x Object |
| owl | Web Ontology Language Document |
| owl | OWL Source Code |
| ox | Ox Program |
| oxo | Ox Compiled Object |
| oxr | OPRO XML Report |
| ozf | Mozart Functor File |
| p | Pascal/Delphi Source Code |
| p | MATLAB Compiled Program |
| p1 | HI-TECH C Compiler Precompiled |
| p4a | Python for Android Script |
| p5 | Perl v5 Script |
| pag | Visual Basic Property Page File |
| page | HybridJava Web Page |
| par | FactoryTalk View Studio Parameter Component File |
| pas | Pascal/Delphi Source Code |
| pas | FreePascal Source Code |
| pasm | Parrot Assembly Source |
| pb | PureBasic Source Code |
| pb1 | OrindaBuild Configuration |
| pb2 | OrindaBuild Configuration |
| pb3 | OrindaBuild Configuration |
| pbc | Parrot Compiled Bytecode |
