| .O | Microsoft Corporation | File sviluppatore | Compiled Object File | Microsoft Visual Studio | Microsoft Corporation |
| .OBE | SourceForge | File sviluppatore | Objeck Executable File | Objeck | SourceForge |
| .OBR | Micro Focus International plc | File sviluppatore | Borland C++ Object Browser Data | Borland C++ | Micro Focus International plc |
| .OCA | Microsoft Corporation | File sviluppatore | Custom Control Library Type File | Microsoft Visual Basic | Microsoft Corporation |
| .OCB | OriginLab | File sviluppatore | Origin Precompiled Origin C Source Code | Origin | OriginLab |
| .OCTEST | Apple | File sviluppatore | Xcode Objective-C Unit Test Bundle | Apple Xcode | Apple |
| .OCX | Microsoft Corporation | File sviluppatore | ActiveX Control | Microsoft Visual Studio | Microsoft Corporation |
| .ODD | Coby Electronics Corporation | File sviluppatore | Coby Voice Recorder Data | Coby Voice Manager Software | Coby Electronics Corporation |
| .ODL | Microsoft Corporation | File sviluppatore | Object Description Language File | Microsoft Visual Studio | Microsoft Corporation |
| .OHI | Borland | File sviluppatore | Delphi OpenHelp Index | Delphi (IDE) | Embarcadero Technologies |
| .OKM | Gian Paolo Saliola | File sviluppatore | OkMap Map Defining File | OkMap | Gian Paolo Saliola |
| .OLT | Innoshock | File sviluppatore | Orbit Downloads List | Orbit Downloader | Innoshock |
| .OM | Petter Strandmark | File sviluppatore | Omicron Compiler | Omicron | Petter Strandmark |
| .OMO | Open Source | File sviluppatore | OMake Object File | OMake | Open Source |
| .OOM | SAP | File sviluppatore | PowerDesigner Object-Oriented Model | Sybase PowerDesigner | SAP |
| .OPR | OPRO Japan Co. | File sviluppatore | OPRO Print Report | OPRO X Browser | OPRO Japan Co. |
| .OPT | LexisNexis | File sviluppatore | Concordance Database File | LexisNexis Concordance | LexisNexis |
| .OPV | Free Software Foundation | File sviluppatore | OpenPanorama Visit Data | OpenPanorama Package Manager | Free Software Foundation |
| .ORC | Voyetra Turtle Beach | File sviluppatore | Voyetra Digital Orchestrator File | Digital Orchestrator | Voyetra Turtle Beach |
| .ORDEREDTEST | Microsoft Corporation | File sviluppatore | Visual Studio Ordered Test File | Microsoft Visual Studio | Microsoft Corporation |
| .OS | Open Source | File sviluppatore | ObjectScript Source File | ObjectDevel | Open Source |
| .OSC | OpenStreetMap | File sviluppatore | OpenStreetMap Change File | Merkaartor | Bart Vanhauwaert, Chris Browet, Ladislav Láska, and others |
| .OW | Open Source | File sviluppatore | Free Pascal 1.0.x Object File | Free Pascal | Open Source |
| .OWL | Interscape | File sviluppatore | OWL Source Code File | OWL | Interscape |
| .OX | Open Source | File sviluppatore | Ox Source Code | OX Professional | Open Source |
di 1 
