| Extension | File Type | Files scanned | Contributor |
| O | ELF Executable and Linkable format (generic) | 61 | |
| O/OBJ/LIB | Intel 80386 Common Object File Format (COFF) object / library | 205 | Joerg Jenderek |
| O/OBJ/LIB | Intel AMD64 Common Object File Format (COFF) object / library | 716 | Joerg Jenderek |
| O/OBJ/LIB | Intel IA64 Common Object File Format (COFF) object / library | 1 | Joerg Jenderek |
| O | Mac OS X Mach-O Universal Object code | 8 | Joerg Jenderek |
| O/DYLIB/BUNDLE | Mac OS X Universal Binary (generic) | 18 | Joerg Jenderek |
| O | i8086 relocatable Object code | 6 | |
| O2C | Objects to See 3D object | 4 | |
| O3M | Ortho 3D Model | 2 | |
| O5C | OpenStreetMap O5c data | 3 | |
| O5M | OpenStreetMap O5m data | 8 | |
| O65 | 6502 binary relocation format (v1) | 3 | |
| O65 | 6502 binary relocation format (v2) | 2 | |
| OAD | Notaro document | 34 | |
| OADX | WP!nnovatio document | 30 | |
| OADX | WP!nnovatio document (UTF-8) | 7 | |
| OAF | Obsidian Asset File game data archive | 5 | |
| OAP/OSP | Outsystems Application/Solution Pack | 20 | |
| OAS | OASIS stream | 97 | |
| OB3 | ORTIM Zeit data | 289 | Scrapie |
| OBB | Touch Detective game data archive | 3 | |
| OBD | Office Binder Document | 10 | |
| OBD | Office Binder Document (97-2000) | 3 | |
| OBF | OBF | 13 | |
| OBF | Open Board Format | 10 | |
| OBF | OsmAnd Binary maps Format | 21 | |
| OBJ | 3D Construction Kit Object | 5 | |
| OBJ | Blender 3D object | 19 | |
| OBJ | CADRazor 3d model | 32 | |
| OBJ/A | Common Object File Format (COFF) Library | 133 | |
| OBJ | Flashback Object | 7 | |
| OBJ | LightWave 3D exported object | 20 | |
| OBJ | MeshLab mesh | 14 | |
| OBJ | Microsoft BASIC PDS 7.x DOS Object module | 60 | |
| OBJ | Microsoft Basic 7.1 compiled object code | 88 | |
| OBJ | Microsoft QuickBASIC 4.0 DOS Object module | 32 | |
| OBJ | Microsoft QuickBASIC 4.5 DOS Object module | 33 | |
| OBJ | Microsoft VBDOS 1.0 compiled object code | 57 | |
| OBJ | OMF - Relocatable Object Module Format | 14 | |
| OBJ | ObEd 3D Object | 20 | |
| OBJ | Oberon V4 Object code | 234 | Romiras |
| OBJ | Oberon-M Object | 13 | |
| OBJ | Opticks Object | 55 | |
| OBJ/BIN | TransFORTH compiled object code | 5 | |
| OBJ | UVMapper object | 6 | |
| OBJ | WDCxAS Object code (v1) | 9 | |
| OBJ | WDCxAS Object code library (v1) | 3 | |
| OBJ | Wavefront Object (created by 3D Max) | 12 | |
| OBJ | Wavefront Object (created by Hexagon) | 12 | |
| OBJ | Wavefront Object (created by VXelements) | 14 | |
| OBJ | Wavefront Object (exported by 3D Builder) | 7 | |
| OBJ | Wavefront Object (generic) | 21 | |
| OBJ | Wavefront Object (mtllib) | 6 | |
| OBJ | Wavefront Object (mtllib, with rem) | 3 | |
| OBJ | World Construction Set Object | 84 | |
| OBJ | X-CAD Modifier Table | 55 | |
| OBJECT | 3-Demon Object | 21 | |
| OBJSTYLE/TCP/TPS | RapidPlan (generic) | 7 | |
| OBK | Chessmaster Opening Book | 2 | |
| OBK | The Chessmaster 4000 opening book | 5 | |
| OBML | Opera Binary Markup Language | 11 | |
| OBML16 | Opera Mini saved page | 15 | |
| OBO | PSI MI format | 40 | |
| OBP | Bryce Object Presets | 41 | |
| OBP/OPK/OSP/OVP | White Birds Productions game data archive | 6 | |
| OBPACK | ObjectBar theme | 4 | |
| OBR | Borland C++ Object Browser data | 4 | |
| OBR | Borland Turbo C Object Browser data (old) | 12 | |
| OBSP | Oberheim SEM V preset | 10 | |
| OBX | Psion Organiser Block data | 2 | |
| OBZ | Open Board Format Zipped | 21 | |
| OC3/OCT | Metashape dense point cloud | 5 | |
| OCB | OpenChrom chromatogram | 2 | |
| OCC | DB/TextWorks Database Terms and Words | 7 | |
| OCD | MetaEdit OODA Class Diagram | 6 | |
| OCD | OCAD map | 8 | |
| OCF | Oberon/F Code File | 57 | Romiras |
| OCI | OpenCanvas Image | 9 | |
| OCIO | OpenColorIO profile | 62 | |
| OCIO | OpenColorIO profile (with rem) | 23 | |
| OCP | OptiCut Panels | 3 | |
| OCRHEX | OcrHex enconded | 6 | |
| OCS | OCS/Fusion Inventory | 14 | |
| OCT | Radiance Octree | 4 | |
| OCX | OLE Custom / ActiveX Control (32bit) | 63 | |
| OCX | OLE Custom Control (16bit) | 38 | |
| ODB | OLEDB (Rev 1) | 6 | |
| ODB | OpenDocument DataBase (generic) | 7 | |
| ODB | OpenDocument Database (ODF 1.1) | 19 | |
| ODB | OpenDocument Database (ODF 1.2) | 42 | |
| ODC | Oberon/F Document | 35 | Romiras |
| ODC | OpenDocument Chart document | 11 | |
| ODCL | Open Dialog Control Language for AutoCAD | 25 | Peter Glasl |
| ODD | Oracle Data base Diagram | 6 | |
| ODEX | Optimized Dalvik Executable | 8 | |
| ODF | OpenDocument Formula | 12 | |
| ODG | OpenDocument Graphics document | 23 | |
| ODICT | ODict Dictionary | 4 | |
| ODL | Microsoft Object Description Language | 14 | |
| ODL | Microsoft Object Description Language (with #) | 14 | |
| ODL | Microsoft Object Description Language (with rem) | 11 | |
| ODL | OneDrive Event Log | 5 | |
| ODM | OpenDocument Master Text document | 8 | |
| ODM | OverDrive Media console media control | 10 | |
| ODP | OpenDocument Presentation | 23 | |
| ODS | OpenDocument Spreadsheet document | 27 | |
| ODT | MindRender VREK Object File Format | 5 | |
| ODT | OpenDocument Text document | 86 | |
| ODV | Ocean Data View data (TXT) | 19 | |
| ODV | Photofinish Output Device info | 21 | |
| ODX | Open Diagnostic Data eXchange | 42 | |
| ODX | Open Diagnostic Data eXchange (UTF-8) | 4 | |
| OEACCOUNT | Windows Mail Account | 15 | |
| OEMODEL | Seene 3d model (v2) | 5 | |
| OEMODEL | Seene 3d model (v3) | 8 | |
| OES | OpenCanvas event data | 3 | |
| OFC | Open Financial Connectivity | 4 | |
| OFF | OFF geometry definition | 6 | |
| OFM | Melco DesignShop Project | 9 | |
| OFM | OmniForm Form | 32 | |
| OFNT | IFF Outline Font | 5 | |
| OFORM | ONLYOFFICE Form for online completion | 295 | Joerg Jenderek |
| OFP | Origin Function Plot | 19 | |
| OFR/OFS | OptimFROG encoded audio | 75 | |
| OFS3/1PSP | Sword Art Online game data archive | 3 | |
| OFT | Outlook Form Template | 57 | |
| OFW | TopLevel Forms Form | 19 | |
| OGF | LEONARD'S Sketch drawing | 21 | |
| OGG | OGG Vorbis audio | 37 | |
| OGG | OGG stream (generic) | 35 | |
| OGL | Actor ObjectGraphics | 6 | |
| OGL | Molden OpenGL model | 2 | |
| OGM | OGG Media stream | 4 | |
| OGMO | Ogmo project | 5 | |
| OGP | PlayStation RSD Object Group (gen) | 4 | |
| OGP | PlayStation RSD Object Group (v3.0) | 2 | |
| OGV/OGG | Ogg Theora video | 14 | |
| OGV/OGG | Ogg Vorbis Skeleton/Video | 11 | |
| OHT | Oracle Help for Java mapping | 10 | |
| OIB | Olympus Image Binary | 13 | |
| OID | Oids galaxy data | 8 | |
| OIF | Olympus Original Imaging Format | 11 | |
| OIL | OpenIL/DevIL raster/animation format | 3 | |
| OIR | Olympus RAW format | 6 | |
| OIR | Olympus RAW format (FluoView) | 5 | |
| OIV | OpenIV mod package | 7 | |
| OJS | OpenMG Jukebox Skin | 31 | |
| OKT/OKTA | Oktalyzer module | 47 | |
| OL | STALCRAFT texture | 5 | |
| OLB | MiNT Object Library | 28 | |
| OLB | OrCAD PSpice Capture Symbols Models | 76 | |
| OLB | PageFocus Objects Library | 10 | |
| OLB | PageFocus Palette | 2 | |
| OLEO | Oleo spreadsheet | 11 | |
| OLF | Overlay Font | 9 | |
| OLF | OyezForms saved rigid form (v9, v10) | 6 | |
| OLK | Soulcalibur III game data archive | 6 | |
| OLK14MESSAGE | Microsoft Outlook for Mac 2011 Message | 4 | |
| OLK15 | Outlook Mac temporary data (generic) | 16 | |
| OLK15MESSAGE | Outlook Mac temporary data (messages) | 12 | |
| OLRW/OLR | Openlab Raw Format | 4 | |
| OLS | WinOLS project | 11 | |
| OLSR | olsrd configuration | 2 | |
| OMA | Open MAp format (experimental) | 2 | |
| OMA | Open MAp format (generic) | 5 | |
| OMA | Open MAp format (v3) | 2 | |
| OMA | Sony OpenMG Audio | 6 | |
| OMA | Sony OpenMG Audio (variant) | 9 | |
| OMF | Onyx Music File module | 24 | |
| OMF/OMFI | Open Media Framework Interchange | 7 | |
| OMF | Open Media Framework Interchange (v2) | 2 | |
| OMF | Open Source Metadata Framework | 11 | |
| OML | OutSystems Modeling Language | 46 | |
| OMOD | OpenMRS Module | 10 | |
| OMP | OpenMusic Patch | 88 | |
| OMR | openMSX replay (ungzipped) | 14 | |
| OMT | AWE Games game data container | 2 | |
| OMX | OMAX Make tool path data | 4 | |
| OMX | OMAX Make tool path data (with rem) | 3 | |
| OMX | Ots Media Library | 4 | |
| ON1 | ON1 Photo RAW | 50 | |
| ONB | OpenModelica NoteBook | 21 | |
| OND | Lotus Notes Encapsulated Memo | 6 | |
| ONE | Microsoft OneNote note | 6 | |
| ONE | SEMONE compressed archive | 10 | |
| ONE | Sequencer One song | 9 | |
| ONEPKG | Microsoft OneNote Package | 10 | |
| ONETOC2 | OneNote table of contents | 9 | |
| ONTX | the Word Encrypted Bible Text Module | 3 | |
| OO3/XML | OmniOutliner document (v3-4) | 11 | |
| OOM | PowerDesigner Model | 32 | |
| OOP | OOP compressed archive | 4 | |
| OOUTLINE | OmniOutliner document (v2) | 19 | |
| OP2 | OP2 bank format | 8 | |
| OP2 | Stanford Research Systems Optimlelt file | 2 | |
| OP3 | Junglevision patch file format | 5 | |
| OPA | Open PAper format | 4 | |
| OPA/OPO/APP | Psion Object/OPL Output | 9 | |
| OPAM | OPAM package info | 15 | |
| OPC | Office Upgrade Control | 8 | |
| OPD | Durango Interferometry data | 18 | |
| OPD | OmniPage Document | 18 | |
| OPD | Wyko OPD binary data | 8 | |
| OPD | Z80onOPD Opus disk image | 5 | |
| OPDX | Dektak OPDx profilometry data | 5 | |
| OPENBUS | Altium OpenBus system document | 102 | |
| OPF | EPUB package document metadata | 17 | |
| OPF | OEB Package Format eBook | 19 | |
| OPF | Obsidium Project File | 6 | .Cryorb |
| OPF | OmniPass encrypted | 4 | |
| OPF | Outforce Packed Content | 2 | |
| OPI | OmniPass exported user | 2 | |
| OPINION | Ghidra Opinion | 27 | |
| OPJ | OrCAD Project | 17 | |
| OPJ | Origin Project | 37 | |
| OPJU | Origin Project (Unicode compliant) | 7 | |
| OPK | Origin Pack game data archive | 7 | |
| OPK | Psion Organiser Pack image | 2 | |
| OPL/OPH/OXH | EPOC OPL source | 10 | |
| OPL | Meridian OPL4 soundbank | 2 | |
| OPLI | OPL3BankEditor instrument format (v1) | 2 | |
| OPLI | OPL3BankEditor instrument format (v2) | 2 | |
| OPLI | OPL3BankEditor instrument format (v3) | 3 | |
| OPML/XML | Outline Processor Markup Language | 37 | |
| OPO | EPOC OPL Object module | 29 | |
| OPPC/OPPS3 | Darksiders game data package | 5 | |
| OPS | Office Profile-Settings (v1.1) | 4 | |
| OPS | Office Profile-Settings (v10.0) | 3 | |
| OPS | Office Profile-Settings (v11.0) | 6 | |
| OPS | Opus Spreadsheet | 3 | |
| OPT | Delphi project Options | 4 | |
| OPT | EDITV settings | 2 | |
| OPT | Holtek HT-IDE3000 Options/settings | 7 | |
| OPT | Neuratron PhotoScore Document | 2 | |
| OPT | Visual C local project settings | 7 | |
| OPTIX | FactoryTalk Optix project | 17 | |
| OPUS | Opus compressed audio | 11 | |
| OPX | EPOC OPL eXtension | 6 | |
| OPX | OrgPlus Organization Chart | 17 | |
| OPY | OptiY Model | 14 | TBeu |
| OR2 | IBM / Lotus Organizer 2 data | 4 | |
| ORA | OpenRaster bitmap | 12 | |
| ORC | Csound Orchestra | 20 | |
| ORC | OpenRocket Component | 60 | |
| ORC | Oracle song | 9 | |
| ORF | Olympus digital camera RAW image (IIRO) | 42 | |
| ORF | Olympus digital camera RAW image (IIRS) | 5 | |
| ORF | Olympus digital camera RAW image (MMOR) | 2 | |
| ORG | Creative Music System Intelligent Organ music | 101 | |
| ORG | GENPO Organ | 9 | |
| ORG | Lotus Organizer data (old) | 6 | |
| ORG | Org-mode | 16 | |
| ORG | Organya 2 module | 38 | |
| ORG | Organya module | 80 | |
| ORGAN | Hauptwerk Organ definition | 9 | |
| ORGAN | Hauptwerk Organ definition (UTF-8) | 5 | |
| ORGAN | Hauptwerk Organ definition (with rem) | 14 | |
| ORGAN | Hauptwerk Organ definition (with rem, UTF-8) | 5 | |
| ORI | EasyCode assembler Original configurations | 18 | |
| ORK | OpenRocket project (XML, v1.x) | 57 | |
| ORK | OpenRocket project (zipped) | 75 | |
| ORP | OpenRGB Profile | 5 | |
| ORT | Oric Raw Tape format | 2 | |
| OS6 | Norton pcAnywhere Online Services list | 2 | |
| OSB | OpenStreetMap Binary map data | 2 | |
| OSB | osu! beatmap storyboard data | 2 | |
| OSC | FL Studio 3x Osc settings | 4 | |
| OSC | OpenStreetMap Change data | 3 | |
| OSC | OptionScope Worksheet | 2 | |
| OSD | Okteta grammar (XML) | 20 | |
| OSDB | osu! Collection Manager collection | 3 | |
| OSEXP/OPENSESAME | OpenSesame Experiment | 29 | |
| OSF | Oberon/F Symbol File | 106 | Romiras |
| OSF | Open Slicer Format | 5 | |
| OSG | OpenSG scene | 16 | |
| OSG | OpenSceneGraph legacy text format | 27 | |
| OSGB | OpenSceneGraph Binary format | 5 | |
| OSGT | OpenSceneGraph Text format | 28 | |
| OSIM | OpenSim project | 19 | |
| OSM | OpenStreetMap XML Data | 20 | |
| OSM | Oxygen Software SMS data (v3) | 3 | |
| OSP | OpenShot Project | 25 | |
| OSP | Orion Sampler sample | 8 | |
| OSP | Synth Pack module | 24 | |
| OSQ | Original Sound Quality audio | 6 | |
| OSS | Open Music System Studio Setup | 2 | |
| OST | Outlook Exchange Offline Storage | 20 | Jonathan Marsh |
| OSU | Osu! script | 4 | |
| OSU | osu! beatmap note timing data (v14) | 21 | |
| OSZ | osu! compressed beatmap data | 15 | |
| OTA | Total Annihilation map data | 87 | |
| OTF | OpenType Font | 11 | |
| OTG | OpenDocument Graphics template | 15 | Joerg Jenderek |
| OTH | OpenDocument HTML Template | 6 | Joerg Jenderek |
| OTM | Tools Media Corp. Mesh | 1395 | |
| OTO | AutoPano project | 2 | |
| OTP | OpenDocument Presentation Template | 30 | |
| OTRKEY | OnlineTVRecorder (OTR) Keyfile | 5 | Icfu |
| OTS | OpenDocument Spreadsheet template | 39 | |
| OTS | OpenTimestamps proof | 8 | |
| OTT | OpenDocument Text Document template | 411 | |
| OTX | Olitext document | 3 | |
| OTX | Oliwrite document | 5 | |
| OTZ | OpenLP Theme | 5 | |
| OUT | GNU gprof performance data | 7 | |
| OUT | Lua 4.0 bytecode | 5 | |
| OUT | Lua 5.0 bytecode | 5 | |
| OUT | Lua 5.1 bytecode | 9 | |
| OUT | Lua 5.2 bytecode | 6 | |
| OUT | Lua 5.3 bytecode | 5 | |
| OUT | Lua bytecode (generic) | 13 | |
| OUT/TXT | Wireshark traffic log | 20 | |
| OUTJOB | Altium Designer Output Job | 21 | |
| OVA | Open Virtualization Format package | 19 | |
| OVD | ObjectVision Datafile | 43 | |
| OVE | Cakewalk Overture Score | 12 | |
| OVF/SVF | OOMMF Vector Field 0.0 / Simple Vector Field | 2 | |
| OVF | OOMMF Vector Field 1.0 format | 5 | |
| OVF | OOMMF Vector Field 2.0 format | 2 | |
| OVF | Open Virtualization Format descriptor | 19 | |
| OVF | Open Virtualization Format descriptor (UTF-8) | 8 | |
| OVL | C-Worthy Machine Dependant Overlay (v1.x) | 7 | |
| OVL | Elite: Dangerous game data | 16 | |
| OVL | Geogrid Overlay (binary) | 6 | |
| OVL | IBM PC Overlay | 4 | |
| OVL | JEMM memory manager | 4 | |
| OVL | Logitech Modula-2 Overlay (v3.x) | 9 | |
| OVL | RollerCoaster Tycoon 3 game data | 3 | |
| OVPN | OpenVPN profile (var.1) | 21 | |
| OVPN | OpenVPN profile (var.2) | 10 | |
| OVPN | OpenVPN profile (with rem) | 11 | |
| OVR | Borland Overlay | 6 | |
| OVR | Borland TLink Overlay | 8 | |
| OVR | NewWord messages Overlay | 3 | |
| OVR | Turbo Pascal Overlay | 5 | |
| OVR | UCDOS Overlay | 3 | |
| OVS | Visi On Overlay | 7 | |
| OVW | Audio Overview | 31 | |
| OVX | Psion OVAL Control | 12 | |
| OWL | OWL XML Ontology | 24 | |
| OWS | OWS (fake) compressed archive | 3 | |
| OWS | OWS compressed disk image | 4 | |
| OXM | FastTracker 2 eXtended Module (Ogg compressed) | 10 | |
| OXO | PhoXo image | 5 | |
| OXP | OmniRush eXtended Package | 6 | |
| OXPS/XPS | Open XML Paper Specification | 66 | |
| OXS | Open Cross Stitch pattern | 6 | |
| OXT | OpenOffice Extension | 13 | Stef |
| OXT | OpenOffice Extension (Dictionary) | 7 | |
| OXYGENE | Oxygene Project | 5 | |
| OXYGENE | Oxygene Project (UTF-8) | 10 | |
| OZB | Mu Online Bitmap | 3 | |
| OZF | Mozart functor | 132 | |
| OZF2 | OziExplorer Map | 11 | |
| OZFX3 | OziExplorer Map | 3 | |
| OZIP | OPPO firmware update (encrypted) | 8 | |
| OZP | Mu Online Navimap | 86 | |
| OZV | ORTIM Zeit project | 6 | Scrapie |