| File extension Name |
extension description |
| .O | UNIX/GCC Object File |
| .O | Free Pascal Object FIle for Linux or DOS Version (FPC Development Team) |
| .O | Object File |
| .O | Origin C Compiled Program File (OriginLab Corporation) |
| .O | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .O | Executable and Linkable Format File |
| .O | Xcode Compiled Core C or Assembler Program (Apple Inc.) |
| .O$$ | Sprint Outfile |
| .O$$ | Output File |
| .O01 | Typhoon Voice File |
| .O1 | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .O1 | Free Pascal Object File for GO321v1 Platform (FPC Development Team) |
| .O2C | Objects to See 3D File Format (mb Software AG) |
| .O3 | Ozone Messenger Program Support Files (htmlland.net) |
| .O8O | Mediator5 Repositorytabs Vorlagen.006 File |
| .OAB | Outlook Address Book (Microsoft Corporation) |
| .OAF | Flight Simulator Texture File (Microsoft Corporation) |
| .OAF | ETH Oberon Applet File |
| .OAF | Outrage Animation Rows |
| .OAH2DSK | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OARC | Optix Archive Bundle (mindwrap, inc.) |
| .OARC | Optix Archive Manifest (mindwrap, inc.) |
| .OAZ | NetFax Manager OAZ Fax File |
| .OB | ABF Outlook Backup Backup File (ABF software, Inc.) |
| .OB | IMB LinkWay Object File |
| .OB! | Orbit Downloader Incomplete Download (OrbitDownloader.com) |
| .OB$ | Compressed OBJ (Microsoft Corporation) |
| .OB1 | OzWin CompuServe E-mail/Forum Access Saved Outbox File |
| .OB3 | ORTIM Zeit Data (ORTIM Industrial Engineering Deutschland GmbH) |
| .OB3 | Outlook Backup Full Backup File (ABF software, Inc.) |
| .OBA | Timespace X100 Digital Video Recorder Video File (Timespace Technology Ltd.) |
| .OBD | Office Binder Document (Microsoft Corporation) |
| .OBD | Oracle Forms Online Help File (Oracle) |
| .OBE | System DLL Oobe File |
| .OBE | Out Of Box Experience |
| .OBJ | LightWave 3D Object (NewTek) |
| .OBJ | Hyperion-CADRazor 3D Model (TITAN Algorithms) |
| .OBJ | Blender 3D Object (The Blender Foundation) |
| .OBJ | UVMapper Object (Stephen L Cox) |
| .OBJ | Wavefront 3D Object |
| .OBJ | Relocatable Object Code |
| .OBJ | Metatools Object Bryce Support File |
| .OBJ0 | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OBJ1 | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OBJECTS | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OBK | Ubi Soft Chessmaster Opening Book File |
| .OBK | Oxygen Phone Manager Phone Book (Oxygen Software) |
| .OBML | Opera Mini Saved Web Page (Opera Software ASA) |
| .OBN | Oberon-2 Module Source File |
| .OBP | Bryce 5 Object Preset (DAZ Productions) |
| .OBPACK | ObjectBar Object Package (Stardock Corporation, Inc.) |
| .OBR | Objective Reference Document Link (Objective Corporation Limited) |
| .OBR | C++ Object Browser Data File (Borland Software Corporation) |
| .OBS | OzWin CompuServe E-mail/Forum Access Saved Outbox File |
| .OBS | ObjectScript Script |
| .OBSKIN | ObjectBar Skin File (Stardock Corporation, Inc.) |
| .OBT | Office Binder Template (Microsoft Corporation) |
| .OBTHEME | ObjectBar Theme File |
| .OBTHEME | ObjectBar Theme File (Stardock Corporation, Inc.) |
| .OBV | ObjectScript Visual Interface |
| .OBX | OpenBook OpenBook 8.0 Document File (Freedom Scientific, Inc) |
| .OBX | Rational XDE (IBM) |
| .OBZ | Office Binder Wizard (Microsoft Corporation) |
| .OC8 | OrgCon 8 Organization (EcoMerc) |
| .OC9 | OrgCon 9 Organization (EcoMerc) |
| .OCA | Control Typelib Cache (Microsoft Corporation) |
| .OCA | OLE Custom Control Library Information |
| .OCA | Visual Basic Control Typelib Cache |
| .OCB | Origin Precompiled Origin C File (OriginLab Corporation) |
| .OCC | OptControlCentre Model File |
| .OCC | DB/TextWorks Database Terms and Words (Inmagic, Inc.) |
| .OCCSAN | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OCD | OCAD Digitized Map (Steinegger Software) |
| .OCD | OrgCon 7 Organization (EcoMerc) |
| .OCE | Brio Query (Hyperion Solutions Corporation) |
| .OCEN | EdGCM Data File (The EdGCM Cooperative Project of Columbia University) |
| .OCF | Object Craft File |
| .OCF | OM2Cipher File |
| .OCI | openCanvas Image (System Products Co., Ltd.) |
| .OCM | Netscape Communicator Aim File |
| .OCM | Internet Odyssey 2 Update |
| .OCM | AOL Advertising Control File (America Online, Inc.) |
| .OCN | OneCable.net - Explorer Measurement Data File (OnTimeLine) |
| .OCP | Offline Commander Project File |
| .OCP | Advanced Art Studio |
| .OCR | FAXGrapper FAX Text File |
| .OCS | eDoc DigiSign Digitally Signed Document (Fujitsu Denmark) |
| .OCSP | Optix Scanner Plug-in (mindwrap, inc.) |
| .OCT | Octalizer Musical File |
| .OCT | Radiance Octree Format |
| .OCTOPUSFONT | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OCX | Object Linking and Embedding (OLE) Control Extension (Microsoft Corporation) |
| .OC_ | Doko Compressed Install File |
| .OD1 | Omnis5 Database File |
| .OD2 | Omnis5 Database File |
| .OD3 | Omnis5 Database File |
| .OD4 | Omnis5 Database File |
| .OD5 | Omnis5 Database File |
| .OD6 | Omnis5 Database File |
| .OD7 | Omnis5 Database File |
| .OD8 | Omnis5 Database File |
| .OD9 | Omnis5 Database File |
| .ODA | ODA Document |
| .ODA | Used with OCX |
| .ODA | AIBO Audio Voice (Sony) |
| .ODA | CALS Group IV Type 2 Raster Image |
| .ODB | OpenOffice/StarOffice OpenDocument (Ver 2) Database (Sun Microsystems, Inc.) |
| .ODB | ABAQUS File (ABAQUS, Inc.) |
| .ODB | Organizers Database (ODB) Database (Organizers' Collaborative) |
| .ODB | ArcView Object Database ASCII File (ESRI) |
| .ODB | Psion Organiser Datafile and LZ Diary File (Psion PLC) |
| .ODC | Office Data Connection |
| .ODC | OpenOffice/StarOffice OpenDocument (Ver 2) Chart (Sun Microsystems, Inc.) |
| .ODCCUBEFILE | Office Data Connection |
| .ODCDATABASEFILE | Office Data Connection |
| .ODCNEWFILE | Office Data Connection |
| .ODCTABLEFILE | Office Data Connection |
| .ODD | Coby Voice Recorder Format (Coby Electronics Corp.) |
| .ODDS | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .ODE | Office Object Data Embedding File (Microsoft Corporation) |
| .ODF | BattleZone Cartographers Guild File |
| .ODF | Open Document Interchange (ODIF) |
| .ODF | OnkoS Data (OnkoS) |
| .ODF | OpenOffice/StarOffice OpenDocument (Ver 2) Formula (Sun Microsystems, Inc.) |
| .ODF | Star Trek Armada Ship/Structure Infomation |
| .ODG | OpenOffice/StarOffice OpenDocument (Ver 2) Graphics Document (Sun Microsystems, Inc.) |
| .ODH | Odigo History File (Comverse, Inc. a part of Comverse Technology) |
| .ODI | OpenOffice/StarOffice OpenDocument (Ver 2) Image (Sun Microsystems, Inc.) |
| .ODIF | Open Document Interchange Format |
| .ODL | Object Definition Language |
| .ODL | Visual C++ Type Library Source (Interface Definition Language File) (Microsoft Corporation) |
| .ODM | OverDrive Media Console Media Control File (OverDrive Inc.) |
| .ODM | Opendisc Software (Opendisc) |
| .ODM | OpenOffice/StarOffice OpenDocument (Ver 2) Global Text Document (Sun Microsystems, Inc.) |
| .ODMP | Optix Collection Package (mindwrap, inc.) |
| .ODOCK | ObjectDock Theme (Stardock Corporation) |
| .ODP | OpenOffice/StarOffice OpenDocument (Ver 2) Presentation (Sun Microsystems, Inc.) |
| .ODP | Analytics Parameters for Analysis Services (Actuate Corporation) |
| .ODP | Oedipus |
| .ODS | Outlook Express Mailbox (Microsoft Corporation) |
| .ODS | NCompass Labs Internet Script |
| .ODS | OpenOffice/StarOffice OpenDocument (Ver 2) Spreadsheet (Sun Microsystems, Inc.) |
| .ODS# | OpenOffice/StarOffice OpenDocument (Ver 2) Open Spreadsheet (Sun Microsystems, Inc.) |
| .ODT | MindRender Virtual Reality Explorer Kit (VREK) Object File Format (Themekit Systems Ltd.) |
| .ODT | OpenOffice/StarOffice OpenDocument (Ver 2) Text Document (Sun Microsystems, Inc.) |
| .ODU | Odigo User File (Comverse, Inc. a part of Comverse Technology) |
| .ODZ | IBM OpenDoc Editor Package File |
| .OEB | Open eBook Document (International Digital Publishing Forum (IDPF)) |
| .OEB | Outlook Express Backup Wizard |
| .OEM | SB_AWE Keys SB_NT File |
| .OEM | Text Listing of Supported OEM Devices for WinNT |
| .OEM | OEM Data Used During Device Install |
| .OF | MicroImages Outline Font (MicroImages, Inc.) |
| .OF | LessDATA Video System OneFile File |
| .OF | Audio Utility Tuningwrench Compressed File |
| .OFASTER | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OFC | Open Financial Connectivity File |
| .OFD | 20-20 Office Sales E-magine File (20-20 Technologies) |
| .OFD | OfficeForms Form Data File (Toplevel Computing) |
| .OFD | ObjectView Form Defintion |
| .OFF | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OFF | Vector Graphic |
| .OFF | 3D Mesh Object File Format |
| .OFFENSE | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OFFICES | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OFL | OtsTurntables Playlist (Ots Labs) |
| .OFM | Font (Adobe Systems Incorporated) |
| .OFM | PostScript Font Description File |
| .OFM | OfficeForms Blank Form File (Toplevel Computing) |
| .OFM | OmniForm Form File (Nuance Communications, Inc.) |
| .OFM | Embroidery Design File |
| .OFN | Systems Management Server (SMS) Offer Creation Notification (Microsoft Corporation) |
| .OFN | Office FileNew File (Microsoft Corporation) |
| .OFR | Systems Management Server (SMS) Offer Description File (Microsoft Corporation) |
| .OFR | OptimFROG Encoded Audio (Florin Ghido) |
| .OFT | Outlook Item Template (Microsoft Corporation) |
| .OFW | Toplevel Forms Form (Toplevel Computing Ltd) |
| .OFX | Open Financial Exchange File |
| .OFX | Olicom Fax |
| .OGA | Ogg Vorbis Audio Profile (Xiph.Org Foundation) |
| .OGF | OnkoS Graphic File (OnkoS) |
| .OGF | Outrage Graphic Rows |
| .OGF2 | MapViewGPS Map Coordinates File |
| .OGF3 | MapViewGPS Map Coordinates File |
| .OGG | Ogg Vorbis Codec Compressed Multimedia File (Xiph.Org Foundation) |
| .OGG | Origin Graphic File (OriginLab Corporation) |
| .OGG | Salt Lake City Game File |
| .OGI | OpenPBX Gateway Interface File (OpenPBX.org) |
| .OGL | Overlay Source Code |
| .OGM | Ogg Vorbis Compressed Video File |
| .OGM | Origin Matrix File (OriginLab Corporation) |
| .OGNC | Web Page Source Code |
| .OGO | Origin Pro Graphic Object File (OriginLab Corporation) |
| .OGS | Origin Script File (OriginLab Corporation) |
| .OGV | Theora Ogg Transport Stream Video File (Xiph.Org Foundation) |
| .OGV | Ogg Vorbis Video Profile (Xiph.Org Foundation) |
| .OGW | Origin Worksheet File (OriginLab Corporation) |
| .OGX | Ogg Vorbis Multiplex Profile (Xiph.Org Foundation) |
| .OGX | C++ Component Gallery Exported Classes and Resources (Borland Software Corporation) |
| .OH | Overhoor ( DrillAssistant) List (Efkasoft) |
| .OH4 | Overhoor ( DrillAssistant) List (Efkasoft) |
| .OHC | OpenHelp Table of Contents File (Borland Software Corporation) |
| .OHI | OpenHelp Index (Borland Software Corporation) |
| .OHL | OpenHelp Link (Borland Software Corporation) |
| .OHP | OpenHelp Project File (Borland Software Corporation) |
| .OHP | DOS 7 File |
| .OHS | Bink Outbound History |
| .OHSD | HealthFrame OpenHealth Service Plug-in (Records For Living, Inc.) |
| .OHW | Overhoor ( DrillAssistant) List (Efkasoft) |
| .OID | ProCurve Manager Non-ProCurve Network Device File (Hewlett-Packard Development Company, L.P.) |
| .OID | Prosa/OM Object Interaction Diagram |
| .OIF | QuickBase Web-sharable Database Interchange File |
| .OIL | Open Image Library Format |
| .OIT | SleepScan and Ceegraph Offset Index File (Bio-logic Systems Corp.) |
| .OJP | 3WayPack (The Three-Mode Company) |
| .OK | Dot OK File Digest (John Oberschelp) |
| .OK | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OKM | OkMap Map Defining File (Gian Paolo Saliola) |
| .OKS | OKScript File (Mike Mardesich) |
| .OKT | Oktalyzer Tracker Module |
| .OKX | OKScript File (Mike Mardesich) |
| .OLB | OrCAD PSpice Capture Symbols Model (Cadence Design Systems, Inc.) |
| .OLB | AUTOGEN |
| .OLB | Automation Object Library (Microsoft Corporation) |
| .OLB | PSpice Capture Symbols Models (Cadence Design Systems, Inc.) |
| .OLB | VAX Object Library |
| .OLB | Medical Terminology Challenge Data File (Delmar Learning) |
| .OLD | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OLD | Old Version |
| .OLDEST | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OLE | Object Linking and Embedding (OLE) Object |
| .OLF | OyezForms Legal Documents (UK) (Oyez) |
| .OLG | OATS Log File (AmsterCHEM) |
| .OLI | Olivetti Text File |
| .OLK | MS Mail Mailbag Lock |
| .OLK | Outlook Address Book (Microsoft Corporation) |
| .OLL | TrialDirector Object Load List |
| .OLL | TrialDirector Load File (inData Corporation) |
| .OLLIEFONT | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OLN | Visual C++ Outline Examples (Microsoft Corporation) |
| .OLR | Openlab Raw Format (Improvision) |
| .OLRW | Openlab Raw Format (Improvision) |
| .OLSR | olsrd Configuration (olsr.org) |
| .OLT | Orbit Downloader List File (OrbitDownloader.com) |
| .OLT | Visual C++ Outline Examples (Microsoft Corporation) |
| .OLV | OnLive! Traveler Sound File (The Digital Space Commons) |
| .OLV | OyezForms Legal Documents (UK) (Oyez) |
| .OM | Omicron Compiler File |
| .OMA | Sony OpenMG Music Format File (Sony Corporation of America) |
| .OMB | Omnibus Offline Web Storage (Alfega Technologies, Inc.) |
| .OMD | Prosa/OM Class Model |
| .OMF | Operation, Management & Finance Database (ENIAC Computing) |
| .OMF | Open Media Format File |
| .OMG | O&O DiskImage Disk Image (O&O Software GmBH) |
| .OMG | OpenMG Jukebox (Sony) |
| .OMN | Omnipotent, Inc. Collection File |
| .OMO | Oracle Media Objects File |
| .OMP | OpenMind Project (MatchWare A/S) |
| .OMS | Briggs Softworks Order Maven |
| .OMS | Omega Downloader Configuration File |
| .OMS | Macintosh MP3 Music Format |
| .OMX | OMAX Make Tool Path File (OMAX Corporation) |
| .OMX | Ots Media Library (Ots Labs Pty Ltd) |
| .ON | Audio Utility Tuningwrench Compressed File |
| .ON | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OND | Notes Database (IBM) |
| .ONE | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .ONE | EdgeCAM Licensing Data Ffile (Pathtrace Engineering Systems) |
| .ONE | SEMONE Compressed Archive |
| .ONE | docPointer Visual ReadMe File (docPointer) |
| .ONE | OneNote Note File (Microsoft Corporation) |
| .ONF | Open Navigation Format File |
| .ONLINE | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .ONX | Onyx Graphics Postershop File |
| .OO | Free Pascal Object File for OS/2 Version (FPC Development Team) |
| .OOM | Shroom Swap File |
| .OOP | OOP Packer Archive (TonyTown Software Ltd.) |
| .OP | Rescue Disk File |
| .OP | Origin Preprocessed Origin C File (OriginLab Corporation) |
| .OP2 | Musplay 1.72 File |
| .OP2 | Outpost 2 Saved Game |
| .OP2 | Stanford Research Systems Optimlelt File (Stanford Research Systems, Inc.) |
| .OP2 | NEi Nastran Binary Results Neutral File (Noran Engineering, Inc.) |
| .OP73 | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OPA | Psion OPL Object File (Psion PLC) |
| .OPC | Beta 44 Job File |
| .OPC | Omron OPC Client File |
| .OPC | Office Upgrade Control File (Microsoft Corporation) |
| .OPD | OmniPage Document (Nuance Communications, Inc.) |
| .OPD | Durango Interferometry Software Document (Diffraction International) |
| .OPEN | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OPENYOURMIND | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OPF | Obsidium Project File (Obsidium Software) |
| .OPF | Flip Album File |
| .OPF | KEPServerEX Project Configuration File (Kepware Technologies Inc.) |
| .OPF | Visual OMP Project File (DNA Software, Inc.) |
| .OPF | Gemstar eBook Publisher Package (Gemstar eBook) |
| .OPI | OmniPass Exported User (Softex, Inc.) |
| .OPI | ExpressRecognition Server OCR Processing Information File (Adlib Software) |
| .OPJ | Origin Project File (OriginLab Corporation) |
| .OPJ | OrCAD Project File (Cadence Design Systems, Inc.) |
| .OPJ | OmniPage Schedule (Nuance Communications, Inc.) |
| .OPK | Origin Origin Pack File (OriginLab Corporation) |
| .OPL | Psion Organiser Programming Language Source File (Psion PLC) |
| .OPM | Quadrox Digital CCTV System Components |
| .OPML | Outline Processor Markup Language |
| .OPN | Strip Poker Deluxe Opponent File (Eagle Rock Enterprises, Inc.) |
| .OPN | Court Opinion File (Tennessee Administrative Office of the Courts) |
| .OPN | Rebel Strip Poker Opponent File (Rebel Software) |
| .OPN | Exact Active Options |
| .OPO | Psion Organiser OPL Code (Psion PLC) |
| .OPO | Psion OPL Output Executable File (Psion PLC) |
| .OPOS | VirtuaSys Opos Script (VirtuaSys :: Sistemas Virtuais) |
| .OPP | IPhoto File (Keronsoft) |
| .OPR | OPRO X Browser Print Report File (OPRO Japan Co.,Ltd.) |
| .OPS | Dynamite VSP Object Placement Style (3am Solutions (UK) Limited) |
| .OPS | Office Profile Settings File (Microsoft Corporation) |
| .OPT | Segue SilkTest Option Set File |
| .OPT | Developer Studio File Workspace Options (Microsoft Corporation) |
| .OPT | Delphi Project Options (Borland Software Corporation) |
| .OPT | Options/Configuration |
| .OPT | QEMM Optimize |
| .OPT | Business Integrator Option File (Pervasive Software Inc.) |
| .OPT | Psion Organiser OPL Source (Psion PLC) |
| .OPT | ProWORX Nxt List Module Option File (Schneider Electric) |
| .OPT | FLEXnet Manager Options File (Acresso Software Inc.) |
| .OPT | pcAnywhere Option Set File |
| .OPT | PhotoScore Document (Neuratron Ltd) |
| .OPT | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OPTIONAL | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OPTNS | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OPUS | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OPW | OrgPlus Organization Chart (HumanConcepts, LLC) |
| .OPX | Psion Organiser Programming Language (Psion PLC) |
| .OPX | OrgChart Organization Diagram (Microsoft Corporation) |
| .OPX | OPL Extension DLL |
| .OPX | Flip Album File |
| .OPX | Exact Inactive Options |
| .OPX | OrgPlus Organization Chart (HumanConcepts, LLC) |
| .OPY | OptiY Model (OptiY) |
| .OQY | Excel OLAP Query File (Microsoft Corporation) |
| .OR | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OR2 | Organizer Ver 2 File (IBM) |
| .OR3 | Organizer 97 File (IBM) |
| .OR4 | Organizer (IBM) |
| .OR5 | Organizer (IBM) |
| .OR6 | Organizer Data File (IBM) |
| .OR8 | Rational Rose Oracle8 Database Model Report (IBM) |
| .ORA | Oracle 7 Configuration |
| .ORC | Oracle 7 Script |
| .ORC | cSound Synthesis Instruments (Csounders) |
| .ORC | Digital Orchestrator Plus Sound File (Voyetra Turtle Beach, Inc.) |
| .ORC | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .ORCA | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .ORD | BC45 Owl File |
| .ORD | OMAX Make Tool Path File (OMAX Corporation) |
| .ORD | Open Reminder Database (ITOpen.eu SOFTWARE) |
| .ORE | Narbacular Drop Game Content (DigiPen Corporation) |
| .ORE | 3WayPack (The Three-Mode Company) |
| .ORF | Descent 3 Outrage Room Format |
| .ORF | Olympus Digital Camera Raw Image File |
| .ORG | Yamaha Organ Flutes (Yamaha Corporation of America) |
| .ORG | Odds Ratio Generator Data File (Psytek Ltd.) |
| .ORG | Hollywood FX Effects Organizer File (Pinnacle Systems, Inc.) |
| .ORG | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .ORG | Microcal Origin Plot |
| .ORG | Netmino File |
| .ORG | Origin File (OriginLab Corporation) |
| .ORG | Organiser (IBM) |
| .ORGAN | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .ORI | Need for Speed-Hot Pursuit 2 File |
| .ORI | Original |
| .ORIG | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .ORIG | Gen Original File |
| .ORIGINAL | Netobjects Fusion Components BBScomp Message File |
| .ORK | Warhammer: Mark of Chaos Game Archive (Games Workshop Group PLC) |
| .ORM | Ormetis Workbench File (Ormetis S.A.S.) |
| .ORMV | Ormetis View File (Ormetis S.A.S.) |
| .ORO | 3WayPack (The Three-Mode Company) |
| .ORV | Oracom Video Format (Oracom Co. Ltd.) |
| .OS | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OS2 | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OS3 | OS/2 Warp 3 File |
| .OSA | W32/Sober-AD |
| .OSB | Sega Dreamcast Sound (One Shot Bank) (Sega of America, Inc.) |
| .OSC | OptionScope Worksheet (Equis International) |
| .OSC | Fruitypro Samples SS_effects File |
| .OSC | Remote Installation Services Client Installation Wizard File (Microsoft Corporation) |
| .OSC | Formula One 2001 Options File |
| .OSD | SoftGrid Program Description (Softricity, Inc.) |
| .OSD | Open Software Description File (Microsoft Corporation) |
| .OSD | Directory Opus Configuration File (GPSoftware) |
| .OSDX | Windows Search Connector (Microsoft Corporation) |
| .OSF | ICZ OriSAFE |
| .OSF | Opera Skin (Opera Software ASA) |
| .OSF | Distribution Bin File |
| .OSF | Virtual Ticket (MetaCommunications) |
| .OSF | Descent 3 Sound File (Outrage Entertainment) |
| .OSG | OpenSceneGraph ASCII Native Format (OSG Community) |
| .OSI | SunNet OSI Description File (Sun) |
| .OSI | Osiris Database (The Shmoo Group) |
| .OSL | OckamSoft Log File (Ockam Instruments, Inc.) |
| .OSMALLER | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OSp | On-Screen Takeoff (On Center Software, Inc.) |
| .OSS | Office Saved Search (Microsoft Corporation) |
| .OST | Outlook Offline Storage File (Microsoft Corporation) |
| .OST | Tivoli Storage Manager File (IBM) |
| .OST | Exchange Offline Cache (Microsoft Corporation) |
| .OST | On-Screen Takeoff Project (On Center Software, Inc.) |
| .OSTANDARD | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OTB | Nokia Phone Graphics Editor (Nokia) |
| .OTB | On-the-air Bitmap |
| .OTC | OpenOffice/StarOffice OpenDocument (Ver 2) Chart Template (Sun Microsystems, Inc.) |
| .OTF | Blu-ray Text Subtitle Font Information (Blu-ray Disc Association) |
| .OTF | OpenOffice/StarOffice OpenDocument (Ver 2) Formula Template (Sun Microsystems, Inc.) |
| .OTF | Open Type Font Format |
| .OTF | OnkoS Text Data File (OnkoS) |
| .OTG | OpenOffice/StarOffice OpenDocument (Ver 2) Graphics Document Template (Sun Microsystems, Inc.) |
| .OTH | OpenOffice/StarOffice OpenDocument (Ver 2) HTML Text Template (Sun Microsystems, Inc.) |
| .OTH | Power Toys Other Folder Explorer Tool |
| .OTH | Origin Graph Properties File (OriginLab Corporation) |
| .OTHERFOLDER | Power Toys Other Folder Explorer Tool |
| .OTHERS | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OTI | OpenOffice/StarOffice OpenDocument (Ver 2) Image Template (Sun Microsystems, Inc.) |
| .OTL | NoteTab Outline |
| .OTL | Z-Soft Type Foundry Font |
| .OTL | Fookes Super NoteTab Template File |
| .OTL | Ventura Publisher's Type Foundry Outline Editor format |
| .OTL | OpenTrap Event Log |
| .OTM | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OTM | Origin Matrix Template File (OriginLab Corporation) |
| .OTM | Outlook VBA Module (Microsoft Corporation) |
| .OTM | Micro Cookbook File (IMSI/Design LLC) |
| .OTM2 | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OTN | OmniPage Training File (Nuance Communications, Inc.) |
| .OTO | Autopano Project (Alexandre Jenny) |
| .OTOSET | OTOsuite Settings (GN Otometrics) |
| .OTP | Top Secret Crypto Gold One Time Pad Key File (TAN$TAAFL Software Company) |
| .OTP | Origin Graphic Template File (OriginLab Corporation) |
| .OTP | OpenOffice/StarOffice OpenDocument (Ver 2) Presentation Template (Sun Microsystems, Inc.) |
| .OTRKEY | OnlineTvRecorder Encoded Video (OnlineTvRecorder.com) |
| .OTS | PSR-9000 (Yamaha Corporation of America) |
| .OTS | OtsJuke Sound File |
| .OTS | OpenOffice/StarOffice OpenDocument (Ver 2) Spreadsheet Template (Sun Microsystems, Inc.) |
| .OTT | OpenOffice/StarOffice OpenDocument (Ver 2) Text Document Template (Sun Microsystems, Inc.) |
| .OTT | Nokia Phone Ring Tone (Nokia) |
| .OTW | Origin Worksheet Template File (OriginLab Corporation) |
| .OTX | Olivetti Olitext Plus File |
| .OU! | OzWin CompuServe E-mail/Forum Access Outbound Messages Online |
| .OUD | Adlib ExpressRecognition Server User Defined Dictionary (Adlib Software) |
| .OUT | Pro/ENGINEER Temporary File (PTC) |
| .OUT | OzWin CompuServe E-mail/Forum Access Outbound Messages |
| .OUT | PSpice Output File (Cadence Design Systems, Inc.) |
| .OUT | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OUT | Outlines or Output File |
| .OUT | NovaBACKUP Backup Set (NovaStor Corporation) |
| .OUT | Wireshark Traffic Log (Gerald Combs) |
| .OUT | NEi Nastran Tabular Output File (Noran Engineering, Inc.) |
| .OUT | MyInfo Documnet/Outline (Milenix Software) |
| .OUT | 3WayPack (The Three-Mode Company) |
| .OUTPUT | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OUX | PCCharge File (VeriFone) |
| .OV | OrgPlus OrgViewer File (HumanConcepts, LLC) |
| .OV | OpenInsight Database Overflow File (Revelation Software) |
| .OV1 | Program File - Overlay |
| .OV2 | TomTom Point of Interest File (TomTom International BV) |
| .OV2 | Program File - Overlay |
| .OV3 | Program File - Overlay |
| .OVA | Sierra Web Studio Graphic |
| .OVA | Web Studio Project Page (Back To The Beach Software, LLC) |
| .OVB | OnkoS Vocabulary Backup File (OnkoS) |
| .OVD | ObjectVision Datafile (Application) (Borland Software Corporation) |
| .OVD | OverDisk Disk Use Database (Elias Fotinis) |
| .OVE | Cakewalk Overture Score (Cakewalk) |
| .OVER | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OVL | PDP-10 Overlay (Pre-Virtual Memory) |
| .OVL | RollerCoaster Tycoon Program Sprite (Infogrames Entertainment SA) |
| .OVL | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OVL | Program File - Overlay |
| .OVM | BHV Tonstudio Platin File |
| .OVR | Program File - Overlay |
| .OVW | Cool Edit Pro Overview File |
| .OVW | DIANA Overview File |
| .OVW | Cubase .WAV File Image (Steinberg) |
| .OVX | Psion Oval Control (Psion PLC) |
| .OW | Free Pascal Object File for Windows Version (FPC Development Team) |
| .OWC | OutWit Hub OutWit Catch db Format (OutWit Technologies) |
| .OWD | OpenWire Project |
| .OWF | OnkoS Wave Data File (OnkoS) |
| .OWG | OutWit Hub OutWit Gear db Format (OutWit Technologies) |
| .OWL | BC45 Owl File |
| .OWLING | Unknown Apple II File (found on Golden Orchard Apple II CD Rom) |
| .OWM | OutWit Hub OutWit Mashup XML Format (OutWit Technologies) |
| .OWS | Web Studio Project File (Back To The Beach Software, LLC) |
| .OX | Ox Object-Oriented Matrix Programming Language File |
| .OXB | OlapX Report Portfolio (OlapX Software) |
| .OXC | OlapX Cube Definition File (OlapX Software) |
| .OXL | OMEGA Product Suite File |
| .OXO | DB-MAIN Compiled Voyager-2 Program |
| .OXO | Ox Object-Oriented Matrix Programming Language File |
| .OXT | OpenOffice.org Extension (OpenOffice.org Community) |
| .OXV | OlapX Local View File (OlapX Software) |
| .OYB | Organize Your Beanie Babies Data File |
| .OYZ | Lotus Approach Alternate dBASE Index |
| .OZF | Mozart Programming System Functor (Mozart Board) |
| .OZF2 | OziExplorer Map (Des & Lorraine Newman) |
| .OZFX3 | OziExplorer Image File (Des & Lorraine Newman) |
| .OZM | Sharp Organizer Memo Bank |
| .OZP | Sharp Organizer Telephone Bank |
| .OZV | ORTIM Zeit Project (ORTIM Industrial Engineering Deutschland GmbH) |