| .o | Object Format |
| .o01 | Typhoon Voice |
| .o2c | Objects To See 3D |
| .o8d | OCTGN Deck |
| .o8g | OCTGN Game Definition |
| .o8o | Mediator 5 Repository Tabs Vorlagen.006 |
| .o8s | OCTGN Set |
| .oab | Microsoft Outlook Offline Address Book Format |
| .oad | Suite Notaro Document |
| .oaf | Microsoft Flight Simulator Texture |
| .oam | Adobe Edge Overlay Format |
| .oap | Samsung Bada Signed Installation Package |
| .oar | OpenSimulator Archive Format |
| .oas | Microsoft Outlook Attachment Sniffer |
| .oat | Google Android RunTime Code |
| .oav | Microsoft Flight Simulator Texture |
| .oaw | OpenArchitectureWare Workflow |
| .oaz | NetFax Manager OAZ Fax Image |
| .ob | IBM LinkWay Object |
| .ob! | Orbit Downloader Incomplete Download Format |
| .ob3 | ABF Software Outlook Backup Backup |
| .ob5 | TopLang OE Backup |
| .obak | WinSQL Offline Backup |
| .obb | Google Android Opaque Binary Blob Format |
| .obd | Microsoft Office Binder Document Format |
| .obf | OsmAnd Offline GPS Vector Maps |
| .obg | ZBrush Mesh |
| .obi | Microsoft Outlook 2007/2010/2013 RSS Subscription Format |
| .obj | 3D Object Format |
| .objx | Metatools Object Bryce Support |
| .obk | OrCAD Capture Design Or Library |
| .obl | TrueSpace |
| .obml | Opera Mini Saved Web Page Format |
| .obml15 | Opera Mini 5.1 Saved Web Page |
| .obp | Bryce Object Format |
| .obpack | ObjectBar Theme Pack |
| .obr | Borland C++ Object Browser Data |
| .obs | WAsP Obstacle List |
| .obt | Openbox Theme Format |
| .obv | 3D Photo Browser Thumbnail Cache |
| .obw | Toontrack Superior Drummer Audio Format |
| .obx | Rational XDE |
| .obyx | Obyx Control |
| .obz | Compressed 3D Object Format |
| .oc3 | OpenCanvas 3 Event Format |
| .oc4 | OpenCanvas 4 Event Format |
| .oc5 | OpenCanvas 5 Event Format |
| .oc9 | OrgCon Organization |
| .oca | Custom Control Library Type Format |
| .ocamlmakefile | Caml Programming Language Makefile |
| .ocb | Origin Precompiled Origin C |
| .occ | Oceanic DataPool Format |
| .ocd | OCAD Digitized Map Format |
| .ocdc | Mathematica Document |
| .ocdf | OneClickDigital Mp3 Audio Book |
| .oce | Open Catalog Extension Connection Format |
| .ocen | EdGCM Data |
| .ocf | OM2 Cipher |
| .ocg | Osprey Custom Graph |
| .oci | OpenCanvas Image Format |
| .ocm | Netscape Communicator Aim |
| .ocp | Advanced Art Studio Image |
| .ocr | FaxGrapper FAX Transcribed Text Format |
| .oct | RADIANCE Octree Format |
| .octest | Xcode Objective-C Unit Test Bundle |
| .ocv | OCV Training Data |
| .ocx | ActiveX Control Format |
| .oc_ | Compressed ActiveX Control |
| .od | Mascotte Mascopt Request Format |
| .od1 | Omnis 5 Database |
| .od2 | Omnis 5 Database |
| .od3 | Omnis Studio Database |
| .od4-9 | Omnis5 Database |
| .odb | ArcView Object Database ASCII Format |
| .odc | OpenOffice.org OpenDocument Format |
| .odccubefile | OpenDocument 1 |
| .odcnewfile | Microsoft Office Data Connection |
| .odctablefile | Microsoft Office |
| .ode | Microsoft Office |
| .odex | Google Android Optimized Executalbe |
| .odf | Open Document Interchange Format (ODIF) |
| .odg | OpenDocument Drawing Format |
| .odh | Microsoft Visual Studio Interface Definition Language |
| .odi | OpenOffice.org Image Format |
| .odif | Open Document Interchange Format |
| .odl | Object Description Language Format |
| .odm | OpenOffice.org OpenDocument Global Text Document Format |
| .odo | ICUBE Online Operating System Write Document Format |
| .odp | OpenOffice.org OpenDocument Presentation Format |
| .ods | OpenDocument Spreadsheet Format |
| .odt | ODF Text Document Format |
| .odt# | OpenOffice.org Locked Document |
| .odtpc | Oracle User Productivity Kit Presentation |
| .odttf | Microsoft Office Document Embedded Font |
| .odv | Ocean Data View Data |
| .odvi | LaTeX |
| .odx | Microsoft BizTalk Server Diagram Format |
| .od_ | Oracle Compressed Driver |
| .oeaccaount | Windows Mail |
| .oeaccount | Windows Mail Account Settings Format |
| .oeb | Open EBook Format |
| .oecl | Open EDA Component Library |
| .oef | Optim Export |
| .oem | OEM Install Data Format |
| .oemigaccount | Outlook Express Temporary |
| .oet | ESignal Order Entry Ticket |
| .oex | Opera Add-on Format |
| .ofa | Kodak OfotoNow |
| .ofb | Oxygen Forensic Suite Backup |
| .ofc | Open Financial Connectivity Format |
| .ofd | ObjectView Form Definition |
| .off | DEC 3D Object File Format |
| .officeui | Microsoft Office UI Customization |
| .offsets | Offset |
| .ofl | OtsTurntables Playlist |
| .ofm | Adobe PostScript Font Description Format |
| .ofn | MS Office File New (Other Office Documents) |
| .ofp | Objecteering Project |
| .ofr | OptimFROG Encoded Audio |
| .ofs | OptimFROG Dualstream |
| .oft | Microsoft Outlook Template Format |
| .ofx | Open Financial Exchange Format |
| .oga | Ogg Vorbis Audio Profile Format |
| .ogd | Oracle Forms Graph |
| .ogf | S.T.A.L.K.E.R. Model Format |
| .ogg | Ogg Vorbis Audio Format |
| .ogl | ATI Overlay Source Code |
| .ogm | Ogg Vorbis Media Format |
| .ognc | Dynamic Web Page Format |
| .ogo | Origin Graphic Object |
| .ogr | OGR C++ Open Source Library |
| .ogs | Origin Script Format |
| .ogv | Ogg Vorbis Video Format |
| .ogw | OriginLab Worksheet |
| .ogx | Ogg Vorbis Multiplex Profile |
| .ogz | Cube 2 Sauerbraten Map Format |
| .oh | DrillAssistant List |
| .oh4 | Overhoor List |
| .ohp | MS-DOS 7 |
| .ohs | Bink Outbound History |
| .ohw | Overhoor DrillAssistant List |
| .oif | QuickBase Web-sharable Database Interchange |
| .oil | Open Image Library Format Image Format |
| .ojp | 3WayPack |
| .ok | Qedit |
| .okm | OkMap Map Defining |
| .okozo | Okozo Desktop Animation |
| .okr | PowerRecover One Key Recovery |
| .oks | OKScript |
| .okt | Oktalyzer Music Module (MOD) Format |
| .okx | OKScript |
| .ol | Valve Hammer Object Layout |
| .ola | Online Access Format |
| .olb | Microsoft Office OLE Object Library Format |
| .olb2 | CanOpener Library |
| .old | Backup Plus Backup Format |
| .ole | Microsoft Object Linking And Embedding |
| .ole2 | Microsoft Office Binary Document Container Format |
| .olf | OyezForms Legal Documents |
| .olk | Microsoft Outlook Address Book Format |
| .olk14category | Microsoft Outlook 2011 For Mac Category Format |
| .olk14contact | Microsoft Outlook 2011 For Mac Contact Format |
| .olk14dbheader | Microsoft Outlook 2011 For Mac Database |
| .olk14event | Outlook Calendar Event Format |
| .olk14folder | Microsoft Outlook 2011 For Mac Folder |
| .olk14group | Outlook Group Format |
| .olk14mailaccount | Microsoft Outlook 2011 For Mac Mail Account Settings Format |
| .olk14message | Microsoft Outlook 2011 For Mac Message Format |
| .olk14msgattach | Microsoft Outlook 2011 For Mac Message Attachment |
| .olk14msgsource | Microsoft Outlook 2011 For Mac Message Source Format |
| .olk14note | Outlook Note Format |
| .olk14pref | Microsoft Outlook 2011 For Mac Preferences Format |
| .olk14recent | Microsoft Outlook 2011 For Mac Recent Address |
| .olk14rule | Microsoft Outlook 2011 For Mac Rule |
| .olk14schedule | Microsoft Outlook 2011 For Mac Schedule |
| .olk14search | Microsoft Outlook 2011 For Mac Saved Search |
| .olk14signature | Microsoft Outlook 2011 For Mac Signature Format |
| .olk14task | Outlook Task Format |
| .olk14uid | Microsoft Outlook 2011 For Mac UID |
| .oll | TrialDirector Load |
| .olm | Microsoft Outlook For Mac Data Format |
| .oln | Microsoft Visual C++ Outline Examples |
| .olproj | Adobe OnLocation |
| .ols | Microsoft Office List Shortcut |
| .olt | Orbit Downloader Download List Format |
| .olv | Roller Coaster Tycoon |
| .oly | AFP Advanced Function Presentation Overlay Resource |
| .oma | Sony OpenMG Music Format |
| .omc | Origin Customized Menu |
| .omcs | Office Manager Access Format |
| .omd | MapInfo Routing J Server Data |
| .omeg | Mathematica For Mac |
| .omf | Open Media Framework Format |
| .omfi | Open Media Framework |
| .omg | O&O DiskImage Backup Format |
| .oml | TracePro Model |
| .omn | Omnipotent Collection |
| .omo | OMake Object Format |
| .omod | Oblivion Mod Manager Mod Format |
| .omp | Office Manager Pro Document Archive |
| .omr | OMR Template |
| .oms | Order Maven |
| .omt | SITH HLA Object Model Template |
| .omx | OtsAV Media Library Information Format |
| .on2 | On2 Encoded FLV |
| .ond | Lotus Notes Format |
| .one | Microsoft OneNote Document Format |
| .onecache | Microsoft OneNote Cache |
| .onepkg | Microsoft OneNote Package Format |
| .onetmp | Microsoft OneNote Temporary |
| .onetoc | Microsoft OneNote Table Of Contents |
| .onetoc2 | Microsoft OneNote Table Of Contents Format |
| .onm | Outdoor Navigator Map |
| .ont | TheWord Module Format |
| .ontx | TheWord Encrypted Module Format |
| .onw | Omron HMI NTST |
| .onx | OnyxGrass Image |
| .oo3 | Omni Group Outliner 3 Format |
| .oog | PyGraph Python Graphics Interface Object Oriented Graphics |
| .oogl | Geomview Object Oriented Graphics Library |
| .ook | Arachne Batch Script Format |
| .oos | OpenOffice.org Spreadsheet |
| .oot | OpenOffice.org Text |
| .op | ESignal OptionsPlus |
| .op2 | Outpost 2 Saved Game |
| .op4 | Nastran Output |
| .opa | Fly! RAW Image Opacity Map |
| .opal | Microsoft Office Customization Tool (OCT) Language-specific Settings |
| .opax | Microsoft Office Customization Tool (OCT) Standard Settings |
| .opb | Bryce Object |
| .opc | Microsoft Office 97 Upgrade Control |
| .opd | Durango Interferometry Software Document Format |
| .opdownload | Opera Next Incomplete Download |
| .opef | OmniPass Encrypted Folder |
| .openbsd | OpenBSD Readme Format |
| .opf | Open Packaging Format |
| .opg | Offline Plan Generation Package |
| .opi | Adlib OCR Processing Information |
| .opj | OrCAD Capture Project Format |
| .opk | Sinking Island/LIle Noyée Game |
| .oplc | Nokia Logo Format |
| .oplm | Outline Processor Markup Language |
| .opml | OPML Outline Processor Markup Language File Format |
| .opn | Court Opinion |
| .opp | IPhotoDVD |
| .oppc | Space Marine Game Resource Package |
| .opq | Eschalon: Book I Game |
| .opr | OPRO X Browser |
| .ops | Microsoft Office Profile Settings Format |
| .opt | MySQL Database Data Format |
| .options | SE-SOFT Configuration Format |
| .opts | Linux Configuration Options Format |
| .opus | Opus Audio Format |
| .opw | OrgPlus For Windows Organization Chart |
| .opx | Microsoft Office Organization Chart File Format |
| .opxt | OrpPlus Template |
| .opy | OptiY Model |
| .op_ | GKSetup Support |
| .oqy | Microsoft Excel OLAP Query Format |
| .or2 | Lotus Organizer 2 Format |
| .or3 | IBM Lotus Organizer 97 Format |
| .or4 | Lotus Organizer Data Format |
| .or5 | IBM Lotus Organizer Format |
| .or6 | Lotus Organizer Data Format |
| .or8 | Rational Rose Oracle8 Database Model Report |
| .ora | Oracle Database Parameter Configuration Format |
| .orb | Original EBook Reader Ebook |
| .orc | CSound Orchestra Midi Format |
| .orderedtest | Microsoft Visual Studio Ordered Test Format |
| .ore | Ore Executable Format |
| .orf | Olympus Digital Camera Raw Image Format |
| .org | Lotus Organiser Format |
| .org2+ | Lotus Organizer |
| .ori | Original Format |
| .orig | Original Format |
| .original | Netobjects Fusion Components Bbscomp Message |
| .original_epub | EPUB Ebook Backup |
| .original_mobi | MOBI Ebook Backup |
| .ork | Warhammer Mark Of Chaos Game Archive |
| .orl | Object-Relational Language |
| .ormdesigner | ORM Designer Project |
| .ors | DSP Group TrueSpeech Audio Format |
| .ort | COREX Orthographic Transcriptions Format |
| .orv | Oracom Video Format |
| .orx | RadiantOne Virtual Directory Server Database Schema |
| .orz | Holic Game |
| .os | ObjectScript Source Format |
| .os2 | Os2 System |
| .os3 | OS/2 Warp 3 |
| .osa | W32/Sober-AD |
| .osas | Apple Script Library Tani Script |
| .osax | FindFile OSAX Mac Format Type |
| .osb | OpenStreetMap Binary Map Format |
| .osbx | OneSafe Exported Backup |
| .osc | Remote Installation Services Client Installation Wizard Format |
| .oscd | TouchOSC Interface Editor Template |
| .osd | Open Software Description |
| .osdx | Microsoft Windows Search Connector Format |
| .osf | Descent 3 Sound |
| .osg | OSGJS JSON Input |
| .osk | Osu! Skin Format |
| .osl | OckamSoft Log |
| .osm | OpenStreetMap Data Format |
| .osp | OpenShot Project Format |
| .osr | Osu! Game Replay Format |
| .oss | Microsoft Office Saved Search Format |
| .ost | Microsoft Outlook Inbox Off-line Folder Format |
| .osu | Osu! Dialog Format |
| .osw | ONESWING Contents |
| .osx | Apple Mac OS PowerPC Executable Format |
| .osz | Osu! Beatmap Format |
| .ot | TheWord Text Module Format |
| .ota | OTA Bitmap Format |
| .otb | Nokia OTA Bitmap Format |
| .otc | OpenDocument Chart Template Format |
| .otd | OpenIV Texture Data |
| .otf | OpenType Font File Format |
| .otg | OpenOffice.org Graphics Document Template |
| .oth | OpenOffice.org OpenDocument HTML Text Template |
| .oti | OpenDocument Image Template Format |
| .otl | Super NoteTab Template Format |
| .otln | Opal Outline Format |
| .otm | Microsoft Outlook Macro Format |
| .otp | OpenOffice.org Presentation Template Format |
| .otr | IRIX Complied FTR |
| .otrkey | OTRKEY Encoded Video Format |
| .ots | OpenOffice.org OpenDocument Spreadsheet Template Format |
| .ott | OpenOffice.org Text Document Template Format |
| .otv | Rotor-Disc OTV Kit OTV Calibration |
| .otx | TheWord Encrypted Module Format |
| .otz | OpenLP Theme Format |
| .out | Output Format |
| .outline | FontTwister Letter Outline |
| .outlook97 | Microsoft Outlook Custom Mail |
| .oux | Bibexcel |
| .ov | OpenInsight Database Overflow |
| .ov1 | Overlay |
| .ov2 | TomTom Navigator Point Of Interest Format |
| .ov3 | Overlay |
| .ov4 | Overlay |
| .ov5 | Overlay |
| .ova | Open Virtual Machine Format |
| .ovd | ObjectVision Data |
| .ove | Overture Music Score Format |
| .ovf | Open Virtualization Format Package |
| .ovg | ObjectVision Converted Graphic |
| .ovh | OMSI Bus Settings |
| .ovl | Overlay Format |
| .ovolog | OoVoo Log |
| .ovp | The Overlay Maker Package Format |
| .ovpn | OpenVPN Config Format |
| .ovr | Overlay Format |
| .ovs | Ovation 1 Style Sheet |
| .ovw | Logic Pro Overview Format |
| .ow | Free Pascal For Windows Object Format |
| .owc | OutWit Hub OutWit Catch Db Format |
| .owg | OutWit Hub OutWit Gear Outwit Thesaurus Db Format |
| .owl | OWL (Obfuscated Weird Language) Source Code Format |
| .owm | OutWit Hub OutWit Mashup XML Format |
| .owr | PeopleSoft Results Archive |
| .ows | Web Studio Project |
| .owt | Ancestry.com OneWorldTree |
| .ox | Ox Object-Oriented Matrix Programming Language |
| .oxd | PS3 Proxy Server |
| .oxf | IBM Rational Rhapsody |
| .oxo | Ox Object-Oriented Matrix Programming Language |
| .oxps | Windows 8 OpenXPS Document Format |
| .oxt | OpenOffice.org Extension Help Format |
| .oxygene | Oxygene Project Format |
| .oyx | IBM Lotus Approach Alternate Database Index Format |
| .oyz | Lotus Approach Alternate DBASE Index |
| .oz | Opera Job Management Compressed Format |
| .ozf2 | OziExplorer Map |
| .ozfx3 | OziExplorer Image |
| .ozj | Mu Online Graphic |
| .ozp | Orgadata LogiKal |
| .ozt | Mu Online Texture |
File extensions are not normally displayed to users. To change this, go to Control Panel, select Appearance and Personalization and Folder Options. Next, select the View and find the option "Hide extensions for known file types". The option should be deselected (cleared) and confirmed with OK.
