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| Extension | File Type Developer | File Category | File Type Description | Software | Software Developer |
|---|---|---|---|---|---|
| .O1 | Open Source | Data Files | Object File For GO321v1 Platform | Free Pascal | Open Source |
| .OBJ | Wavefront Technologies | 3D Image Files | Wavefront 3D Object File | MeshLab | Open Source |
| .OBML | Opera Software | Web Files | Opera Mini Saved Web Page | MicroEmulator | Open Source |
| .OBML16 | Opera Software | Web Files | Opera Mini Saved Web Page | MicroEmulator | Open Source |
| .OBS | Department of Wind Energy at the Technical University of Denmark, Denmark | Data Files | WAsP Obstacle List Data | Open Broadcaster Software | Open Source |
| .OBT | Open Source | Settings Files | Openbox Theme File | Openbox | Open Source |
| .OCSP | Open Source | Data Files | HAProxy OCSP Data File | HAProxy | Open Source |
| .OCT | Radiance Community | 3D Image Files | Radiance Octree File | RADIANCE | Open Source |
| .ODD | Coby Electronics Corporation | Audio Files | Coby Voice Recorder Data | TEI Tools | Open Source |
| .ODF | Oracle Corporation | Uncommon Files | OpenDocument Formula | AbiWord | Open Source |
| .ODM | Open Source | Text Files | OpenDocument Master Document | AbiWord | Open Source |
| .ODR | Open Source | Data Files | OpenIV Model Data | OpenIV | Open Source |
| .ODT | Oracle Corporation | Text Files | OpenDocument Text Document | AbiWord | Open Source |
| .OEB | IDPF (International Digital Publishing Forum) | eBook Files | Open eBook File | Calibre | Open Source |
| .OEM | Microsoft Corporation | Settings Files | Original Equipment Manufacturer Setup File | OEM Information Editor | Open Source |
| .OFF | Open Source | 3D Image Files | Object File Format | MeshLab | Open Source |
| .OFX | Intuit, Inc. | Data Files | Open Financial Exchange File | GnuCash | Open Source |
| .OGA | Xiph.Org | Audio Files | Ogg Vorbis Audio File | IrfanView | Open Source |
| .OGG | Xiph.Org | Audio Files | Ogg Vorbis Audio File | IrfanView | Open Source |
| .OGX | Xiph.Org | Video Files | Ogg Vorbis Multiplexed Media File | XMMS | Open Source |
| .OIV | Open Source | Plugin Files | OpenIV Mod Package File | OpenIV | Open Source |
| .OKT | Open Source | Audio Files | Oktalyzer Module | ModPlug Player | Open Source |
| .OMO | Open Source | Developer Files | OMake Object File | OMake | Open Source |
| .OO | Open Source | Uncommon Files | Free Pascal Object File For OS/2 Version | Free Pascal | Open Source |
| .OOA | Sublime HQ Pty Ltd | Data Files | OOA Interchange Format Data | PSPad | Open Source |
| .OP | Open Source | Data Files | OpenPAT Report | OpenPAT | Open Source |
| .OPML | The Omni Group | Web Files | Outline Processor Markup Language File | OPML Editor | Open Source |
| .OPT | LexisNexis | Database Files | Concordance Database File | AGiliTy | Open Source |
| .OPUS | Open Source | Audio Files | Opus Audio File | MediaInfo Player | Open Source |
| .OS | Open Source | Developer Files | ObjectScript Source File | ObjectDevel | Open Source |
| .OSC | OpenStreetMap | GIS Files | OpenStreetMap Change File | osmchange | Open Source |
| .OSZ | Ppy Pty Ltd | Game Files | Osu! Beatmap File | opsu! | Open Source |
| .OTD | Open Source | Game Files | OpenIV Texture Data | OpenIV | Open Source |
| .OTF | Open Source | Font Files | OpenType Font | FontForge | Open Source |
| .OTRKEY | OnlineTvRecorder.com (OTR) | Video Files | OnlineTvRecorder Encoded Video File | otrtool | Open Source |
| .OUT | Open Source | Executable Files | Compiled Executable File | Linux | Open Source |
| .OW | Open Source | Developer Files | Free Pascal 1.0.x Object File | Free Pascal | Open Source |
| .OX | Open Source | Developer Files | Ox Source Code | OX Professional | Open Source |
| .OXO | Open Source | Settings Files | Ox Object-oriented Matrix Programming Language Source Code | OX Professional | Open Source |
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