| Extension | Information |
| O | Object File |
| O | UNIX/GCC Object File |
| O | Origin C Compiled Program File (OriginLab) |
| O | Free Pascal Object FIle for Linux or DOS Version (FPC Development Team) |
| O$$ | Output File |
| O$$ | Sprint Outfile |
| O01 | Typhoon Voice File |
| 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) |
| OAF | ETH Oberon Applet File |
| OAF | Outrage Animation Rows |
| OAF | Flight Simulator Texture File (Microsoft) |
| OAZ | NetFax Manager OAZ Fax File |
| OB | IMB LinkWay Object File |
| OB | ABF Outlook Backup Backup File (ABF software, Inc.) |
| OB$ | Compressed OBJ (Microsoft) |
| OB1 | OzWin CompuServe E-mail/Forum Access Saved Outbox File |
| OBA | Timespace X100 Digital Video Recorder Video File (Electronics Services Agency) |
| OBD | Office Binder Document (Microsoft) |
| OBD | Oracle Forms Online Help File (Oracle) |
| OBE | Out Of Box Experience |
| OBE | System DLL Oobe File |
| OBJ | Metatools Object Bryce Support File |
| OBJ | Relocatable Object Code |
| OBJ | Wavefront 3D Object |
| OBJ | LightWave 3D Object (NewTek) |
| OBK | Ubi Soft Chessmaster Opening Book File |
| OBK | Oxygen Phone Manager Phone Book (Oxygen Software) |
| OBN | Oberon-2 Module Source File |
| OBP | Bryce 5 Object Preset (Corel Corporation) |
| OBPACK | ObjectBar Object Package (Stardock Corporation, Inc.) |
| OBR | C++ Object Browser Data File (Borland Software Corporation) |
| OBR | Objective Reference Document Link (Objective Corporation Limited) |
| OBS | ObjectScript Script |
| OBS | OzWin CompuServe E-mail/Forum Access Saved Outbox File |
| OBSKIN | ObjectBar Skin File (Stardock Corporation, Inc.) |
| OBT | Office Binder Template (Microsoft) |
| OBTHEME | ObjectBar Theme File |
| OBTHEME | ObjectBar Theme File (Stardock Corporation, Inc.) |
| OBV | ObjectScript Visual Interface |
| OBX | Rational XDE (IBM) |
| OBZ | Office Binder Wizard (Microsoft) |
| OCA | Control Typelib Cache (Microsoft) |
| OCA | OLE Custom Control Library Information |
| OCA | Visual Basic Control Typelib Cache |
| OCB | Origin Precompiled Origin C File (OriginLab Corporation) |
| OCC | DB/TextWorks Database Terms and Words |
| OCC | OptControlCentre Model File |
| OCD | OCAD Digitized Map (Steinegger Software) |
| OCE | Brio Query (Hyperion Solutions Corporation) |
| OCF | Object Craft File |
| OCF | OM2Cipher File |
| OCM | AOL Advertising Control File (America Online, Inc.) |
| OCM | Internet Odyssey 2 Update |
| OCM | Netscape Communicator Aim File |
| OCP | Advanced Art Studio |
| OCP | Offline Commander Project File |
| OCR | FAXGrapper FAX Text File |
| OCS | eDoc DigiSign Digitally Signed Document (Fujitsu Denmark) |
| OCT | Octalizer Musical File |
| OCT | Radiance Octree Format |
| OCX | Object Linking and Embedding (OLE) Control Extension (Microsoft) |
| 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 | CALS Group IV Type 2 Raster Image |
| ODA | ODA Document |
| ODA | Used with OCX |
| ODA | AIBO Audio Voice (Sony) |
| ODB | ArcView Object Database ASCII File (ESRI) |
| ODB | Psion Organiser Datafile and LZ Diary File (Psion PLC) |
| ODB | OpenOffice/StarOffice OpenDocument (Ver 2) Database (Sun Microsystems, Inc.) |
| ODB | ABAQUS File (ABAQUS, Inc.) |
| ODB | Organizers Database (ODB) Database (Organizers' Collaborative) |
| 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 |
| ODE | Office Object Data Embedding File (Microsoft) |
| ODF | BattleZone Cartographers Guild File |
| ODF | OnkoS Data (OnkoS) |
| ODF | Open Document Interchange (ODIF) |
| ODF | Star Trek Armada Ship/Structure Infomation |
| ODF | OpenOffice/StarOffice OpenDocument (Ver 2) Formula (Sun Microsystems, Inc.) |
| 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) |
| ODM | Opendisc Software (Opendisc) |
| ODM | OpenOffice/StarOffice OpenDocument (Ver 2) Global Text Document (Sun Microsystems, Inc.) |
| ODP | Oedipus |
| ODP | OpenOffice/StarOffice OpenDocument (Ver 2) Presentation (Sun Microsystems, Inc.) |
| ODS | NCompass Labs Internet Script |
| ODS | Outlook Express Mailbox (Microsoft) |
| ODS | OpenOffice/StarOffice OpenDocument (Ver 2) Spreadsheet (Sun Microsystems, Inc.) |
| ODT | OpenOffice/StarOffice OpenDocument (Ver 2) Text Document (Sun Microsystems, Inc.) |
| ODT | MindRender Virtual Reality Explorer Kit (VREK) Object File Format (Themekit Systems Ltd.) |
| ODU | Odigo User File (Comverse, Inc. a part of Comverse Technology) |
| ODZ | IBM OpenDoc Editor Package File |
| OEB | Outlook Express Backup Wizard |
| OEM | OEM Data Used During Device Install |
| OEM | SB_AWE Keys SB_NT File |
| OEM | Text Listing of Supported OEM Devices for WinNT |
| OF | Audio Utility Tuningwrench Compressed File |
| OF | LessDATA Video System OneFile File |
| OF | MicroImages Outline Font (MicroImages, Inc.) |
| OFC | Open Financial Connectivity File |
| OFD | ObjectView Form Defintion |
| OFD | OfficeForms Form Data File (Toplevel Computing) |
| OFF | 3D Mesh Object File Format |
| OFF | Vector Graphic |
| OFM | Embroidery Design File |
| OFM | Font (Adobe) |
| OFM | OmniForm Form File |
| OFM | PostScript Font Description File |
| OFM | OfficeForms Blank Form File (Toplevel Computing) |
| OFN | Office FileNew File (Microsoft) |
| OFR | OptimFROG Encoded Audio (Florin Ghido) |
| OFT | Outlook Item Template (Microsoft) |
| OFX | Olicom Fax |
| OFX | Open Financial Exchange File |
| OGF | OnkoS Graphic File (OnkoS) |
| OGF | Outrage Graphic Rows |
| OGG | Ogg Vorbis Codec Compressed WAV File |
| OGG | Origin Graphic File (OriginLab) |
| OGG | Salt Lake City Game File |
| OGL | Overlay Source Code |
| OGM | Ogg Vorbis Compressed Video File |
| OGM | Origin Matrix File (OriginLab) |
| OGO | OriginPro Graphic Object File (OriginLab) |
| OGS | Origin Script File (OriginLab) |
| OGW | Origin Worksheet File (OriginLab) |
| 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 | DOS 7 File |
| OHP | OpenHelp Project File (Borland Software Corporation) |
| OHS | Bink Outbound History |
| OHW | Overhoor ( DrillAssistant) List (Efkasoft) |
| OID | Prosa/OM Object Interaction Diagram |
| OIF | QuickBase Web-sharable Database Interchange File |
| OIK | Lenné3D Plant Distribution File (Lenné3D GmbH i.G.) |
| OIL | Open Image Library Format |
| OIX | Lenné3D Plant Distribution File (Lenné3D GmbH i.G.) |
| OJP | 3WayPack (The Three-Mode Company) |
| OKS | OKScript File (Mike Mardesich) |
| OKT | Oktalyzer Tracker Module |
| OKX | OKScript File (Mike Mardesich) |
| OLB | AUTOGEN |
| OLB | Automation Object Library (Microsoft Corporation) |
| OLB | PSpice Capture Symbols Models (OrCAD) |
| OLB | VAX Object Library |
| OLB | Medical Terminology Challenge Data File (Delmar Learning) |
| OLD | Old Version |
| OLE | Object Linking and Embedding (OLE) Object |
| OLF | OyezForms Legal Documents (UK) (Oyez) |
| OLI | Olivetti Text File |
| OLK | MS Mail Mailbag Lock |
| OLK | Outlook Address Book (Microsoft) |
| OLL | TrialDirector Object Load List |
| OLN | Visual C++ Outline Examples (Microsoft) |
| OLT | Visual C++ Outline Examples (Microsoft) |
| OLV | OnLive! Traveler Sound File (The Digital Space Commons) |
| OLV | OyezForms Legal Documents (UK) (Oyez) |
| OM | Omicron Compiler File |
| OMA | Sony Music Format File (Sony) |
| OMD | Prosa/OM Class Model |
| OMF | Open Media Format File |
| OMG | OpenMG Jukebox (Sony) |
| OMN | Omnipotent, Inc. Collection File |
| OMO | Oracle Media Objects File |
| OMS | Briggs Softworks Order Maven |
| OMS | Macintosh MP3 Music Format |
| OMS | Omega Downloader Configuration File |
| OMX | OMAX Make Tool Path File (OMAX Corporation) |
| OMX | Ots Media Library (Ots Labs Pty Ltd) |
| ON | Audio Utility Tuningwrench Compressed File |
| OND | Notes Database (IBM) |
| ONE | docPointer Visual ReadMe File (docPointer) |
| ONE | OneNote Note File (Microsoft) |
| ONE | EdgeCAM Licensing Data Ffile (Pathtrace Engineering Systems) |
| ONF | Open Navigation Format File |
| 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 |
| OPA | Psion OPL Object File (Psion PLC) |
| OPC | Beta 44 Job File |
| OPC | Office Upgrade Control File (Microsoft) |
| OPC | Omron OPC Client File |
| OPD | OmniPage Document (ScanSoft, Inc.) |
| OPD | Durango Interferometry Software Document (Diffraction International) |
| OPF | Flip Album File |
| OPF | KEPServerEX Project Configuration File (Kepware Technologies Inc.) |
| OPF | Gemstar eBook Publisher Package (Gemstar eBook) |
| OPF | Obsidium Project File (Obsidium Software) |
| OPI | OmniPass Exported User (Softex, Inc.) |
| OPJ | OrCad Project File |
| OPJ | Origin Project File (OriginLab) |
| OPJ | OmniPage Schedule (ScanSoft, Inc.) |
| OPK | Origin Origin Pack File (OriginLab) |
| OPL | Psion Organiser Programming Language Source File (Psion PLC) |
| OPM | Quadrox Digital CCTV System Components |
| OPN | Exact Active Options |
| OPN | Court Opinion File (Tennessee Administrative Office of the Courts) |
| OPN | Rebel Strip Poker Opponent File (Rebel Software) |
| OPN | Strip Poker Deluxe Opponent File (Eagle Rock Enterprises, Inc.) |
| OPO | Psion OPL Output Executable File (Psion PLC) |
| OPO | Psion Organiser OPL Code (Psion PLC) |
| OPP | IPhoto File (Keronsoft) |
| OPS | Office Profile Settings File (Microsoft) |
| OPS | Dynamite VSP Object Placement Style (3am Solutions (UK) Limited) |
| OPT | Delphi Project Options (Borland Software Corporation) |
| OPT | Developer Studio File Workspace Options (Microsoft) |
| OPT | Options/Configuration |
| OPT | ProWORX Nxt List Module Option File (Schneider Electric) |
| OPT | QEMM Optimize |
| OPT | Segue SilkTest Option Set File |
| OPT | Psion Organiser OPL Source (Psion PLC) |
| OPT | pcAnywhere Option Set File |
| OPT | Business Integrator Option File (Pervasive Software Inc.) |
| OPW | OrgPlus Organization Chart (HumanConcepts, LLC) |
| OPX | OrgPlus Organization Chart (HumanConcepts, LLC) |
| OPX | Exact Inactive Options |
| OPX | Flip Album File |
| OPX | OPL Extension DLL |
| OPX | OrgChart Organization Diagram (Microsoft) |
| OPX | Psion Organiser Programming Language (Psion PLC) |
| OQY | Excel OLAP Query File (Microsoft) |
| 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 | cSound Synthesis Instruments (Csounders) |
| ORC | Oracle 7 Script |
| ORC | Digital Orchestrator Plus Sound File (Voyetra Turtle Beach, Inc.) |
| ORD | BC45 Owl File |
| ORD | OMAX Make Tool Path File (OMAX Corporation) |
| ORE | 3WayPack (The Three-Mode Company) |
| ORF | Descent 3 Outrage Room Format |
| ORF | Olympus Digital Camera Raw Image File |
| ORG | Organiser (IBM) |
| ORG | Microcal Origin Plot |
| ORG | Netmino File |
| ORG | Origin File |
| ORG | Hollywood FX Effects Organizer File (Pinnacle Systems, Inc.) |
| ORI | Need for Speed-Hot Pursuit 2 File |
| ORI | Original |
| ORIG | Gen Original File |
| ORIGINAL | Netobjects Fusion Components BBScomp Message File |
| ORO | 3WayPack (The Three-Mode Company) |
| OS3 | OS/2 Warp 3 File |
| OSC | Formula One 2001 Options File |
| OSC | Fruitypro Samples SS_effects File |
| OSC | Remote Installation Services Client Installation Wizard File (Microsoft) |
| OSD | Open Software Description File (Microsoft) |
| OSD | SoftGrid Program Description (Softricity, Inc.) |
| OSF | Distribution Bin File |
| OSF | ICZ OriSAFE |
| OSF | Descent 3 Sound File (Outrage Entertainment) |
| OSF | Opera Skin (Opera Software ASA) |
| OSG | OSG321 Software Application Data File |
| OSI | Osiris Database (The Shmoo Group) |
| OSI | SunNet OSI Description File (Sun) |
| OSL | OckamSoft Log File (Ockam Instruments, Inc.) |
| OSS | Office Saved Search (Microsoft) |
| OST | Exchange Offline File (Microsoft) |
| OST | Outlook Offline File (Microsoft) |
| OST | Tivoli Storage Manager File (IBM) |
| OTB | Nokia Phone Graphics Editor (Nokia) |
| OTB | On-the-air Bitmap |
| OTC | OpenOffice/StarOffice OpenDocument (Ver 2) Chart Template (Sun Microsystems, Inc.) |
| OTF | OnkoS Text Data File (OnkoS) |
| OTF | Open Type Font Format |
| OTF | OpenOffice/StarOffice OpenDocument (Ver 2) Formula Template (Sun Microsystems, Inc.) |
| OTG | OpenOffice/StarOffice OpenDocument (Ver 2) Graphics Document Template (Sun Microsystems, Inc.) |
| OTH | Power Toys Other Folder Explorer Tool |
| OTH | Origin Graph Properties File (OriginLab Corporation) |
| OTH | OpenOffice/StarOffice OpenDocument (Ver 2) HTML Text Template (Sun Microsystems, Inc.) |
| OTHERFOLDER | Power Toys Other Folder Explorer Tool |
| OTI | OpenOffice/StarOffice OpenDocument (Ver 2) Image Template (Sun Microsystems, Inc.) |
| OTL | Fookes Super NoteTab Template File |
| OTL | NoteTab Outline |
| OTL | OpenTrap Event Log |
| OTL | Ventura Publisher's Type Foundry Outline Editor format |
| OTL | Z-Soft Type Foundry Font |
| OTM | Origin Matrix Template File (OriginLab) |
| OTM | Outlook VBA Module (Microsoft) |
| OTM | IMSI Micro Cookbook File (IMSI) |
| OTN | OmniPage Training File (ScanSoft, Inc.) |
| OTP | Origin Graphic Template File (OriginLab) |
| OTP | Top Secret Crypto Gold One Time Pad Key File (TAN$TAAFL Software Company) |
| OTP | OpenOffice/StarOffice OpenDocument (Ver 2) Presentation Template (Sun Microsystems, Inc.) |
| OTS | OtsJuke Sound File |
| OTS | OpenOffice/StarOffice OpenDocument (Ver 2) Spreadsheet Template (Sun Microsystems, Inc.) |
| OTT | Nokia Phone Ring Tone (Nokia) |
| OTT | OpenOffice/StarOffice OpenDocument (Ver 2) Text Document Template (Sun Microsystems, Inc.) |
| OTW | Origin Worksheet Template File (OriginLab) |
| OTX | Olivetti Olitext Plus File |
| OU! | OzWin CompuServe E-mail/Forum Access Outbound Messages Online |
| OUT | Outlines or Output File |
| OUT | OzWin CompuServe E-mail/Forum Access Outbound Messages |
| OUT | Pro/ENGINEER Temporary File (PTC) |
| OUT | NovaBACKUP Backup Set (NovaStor Corporation) |
| OUT | MyInfo Documnet/Outline (Milenix Software) |
| OUT | 3WayPack (The Three-Mode Company) |
| OUV | Sphinx Open Ended Responses (Le Sphinx Développement) |
| OV | OpenInsight Database Overflow File (Revelation Software) |
| OV | OrgPlus OrgViewer File (HumanConcepts, LLC) |
| OV1 | Program File - Overlay |
| OV2 | Program File - Overlay |
| OV2 | TomTom Point of Interest File (TomTom International BV) |
| 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) |
| OVE | Cakewalk Overture Score (Cakewalk) |
| OVL | Program File - Overlay |
| OVL | RollerCoaster Tycoon Program Sprite (Atari) |
| OVLDEF | Gestionnaire d'extensions Overload Definition (Définition de surcharge) (Francis Gagné) |
| OVM | BHV Tonstudio Platin File |
| OVR | Program File - Overlay |
| OVW | Cool Edit Pro Overview File |
| OVW | Cubase .WAV File Image (Steinberg) |
| OVW | DIANA Overview File |
| OVX | Psion Oval Control (Psion PLC) |
| OW | Free Pascal Object File for Windows Version (FPC Development Team) |
| OWD | OpenWire Project |
| OWF | OnkoS Wave Data File (OnkoS) |
| OWL | BC45 Owl File |
| 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 |
| OXV | OlapX Local View File (OlapX Software) |
| OYB | Organize Your Beanie Babies Data File |
| OYZ | Lotus Approach Alternate dBASE Index |
| OZF2 | OziExplorer Map (Des & Lorraine Newman) |
| OZFX3 | OziExplorer Image File (Des & Lorraine Newman) |
| OZM | Sharp Organizer Memo Bank |
| OZP | Sharp Organizer Telephone Bank |