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