| .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 |
