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