| O File Extensions |
File types with extensions that start with the letter O.
| Extension | File Type | File Summary |
| .o | Manufacturer | Compiled Object File |
| .oab | Data | Offline Address Book |
| .oar | Compressed | OpenSimulator Archive |
| .ob! | Misc | Orbit Downloader Incomplete Download |
| .obi | Settings | Outlook RSS Subscription File |
| .obj | Data | Relocatable Object Code |
| .obj | 3D Image | 3D Object File |
| .obml | Web | Opera Mini Saved Web Page |
| .obp | 3D Image | Bryce Object File |
| .obs | Executable | ObjectScript Script File |
| .obz | 3D Image | Compressed 3D Object File |
| .oca | Manufacturer | Custom Control Library Type File |
| .oce | Settings | Open Catalog Extension Connection File |
| .ocr | Text | FAXGrapper Fax Text File |
| .ocx | Manufacturer | ActiveX Control |
| .od | Data | Mascopt Graph Request File |
| .odb | Database | OpenOffice.org Base Database |
| .odb | Database | ABAQUS Database |
| .odc | Data | OpenDocument Chart File |
| .odc | Settings | Office Data Connection File |
| .odf | Data | Open Document File |
| .odf | Misc | OpenDocument Formula |
| .odf | Game | Battlezone Object Definition File |
| .odg | Vector Image | OpenDocument Graphic File |
| .odi | Raster Image | OpenDocument Image |
| .odif | Text | Open Document Interchange Format |
| .odm | Audio | OverDrive Media File |
| .odm | Text | OpenDocument Master Document |
| .odo | Text | Online Operating System Write Document |
| .odp | Data | OpenDocument Presentation |
| .ods | Misc | Outlook Express 5 Mailbox |
| .ods | Spreadsheet | OpenDocument Spreadsheet |
| .odt | Text | OpenDocument Text Document |
| .odx | Data | BizTalk Server Orchestration File |
| .oeaccount | Data | Windows Mail Account File |
| .oeb | Data | Open eBook File |
| .oeb | Backup | Outlook Express Backup Wizard File |
| .oem | Settings | OEM Setup File |
| .oex | Plugin | Opera Extension |
| .ofc | Data | Open Financial Connectivity File |
| .off | 3D Image | Object File Format |
| .ofl | Text | Ots File List |
| .ofm | Data | OmniForm Form |
| .oft | Data | Outlook File Template |
| .ofx | Data | Open Financial Exchange File |
| .oga | Audio | Ogg Vorbis Audio File |
| .ogf | 3D Image | S.T.A.L.K.E.R. Model File |
| .ogg | Audio | Ogg Vorbis Compressed Audio File |
| .ogm | Video | Ogg Media File |
| .ognc | Web | Dynamic Web Page |
| .ogv | Video | Ogg Vorbis Video File |
| .ogx | Video | Ogg Vorbis Multiplexed Media File |
| .ogz | Game | Cube 2 Map File |
| .okt | Audio | Oktalyzer Module |
| .ola | Page Layout | Online Access File |
| .olb | Misc | OLE Object Library |
| .old | Backup | Backup File |
| .olk | Data | Outlook Address Book File |
| .olk14category | Data | Outlook Category File |
| .olk14contact | Data | Outlook Contact File |
| .olk14event | Data | Outlook Calendar Event File |
| .olk14group | Data | Outlook Group File |
| .olk14mailaccount | Data | Outlook Mail Account File |
| .olk14message | Data | Outlook Message File |
| .olk14msgsource | Data | Outlook Email Message Data File |
| .olk14note | Data | Outlook Note File |
| .olk14pref | Settings | Outlook Preferences File |
| .olk14signature | Data | Outlook Signature File |
| .olk14task | Data | Outlook Task File |
| .oma | Audio | Sony OpenMG Music File |
| .omcs | Data | Office Manager Access File |
| .omf | Audio | Open Media Framework File |
| .omf | Raster Image | OMF Interchange Image File |
| .omg | Audio | OpenMG Audio File |
| .omo | Manufacturer | OMake Object File |
| .omp | Data | Office Manager Document Archive |
| .omx | Audio | OtsAV Media Library Information File |
| .ond | Data | Lotus Notes Encapsulated Memo File |
| .one | Data | OneNote Document |
| .onepkg | Backup | Microsoft OneNote Package File |
| .onetoc2 | Settings | Microsoft OneNote Table of Contents File |
| .oo3 | Data | OmniOutliner 3 File |
| .opd | Page Layout | OmniPage Document |
| .openbsd | Text | OpenBSD Readme File |
| .opf | Data | FlipViewer FlipBook File |
| .opf | Misc | Open Packaging Format File |
| .opf | Misc | FlipAlbum File |
| .opml | Web | Outline Processor Markup Language File |
| .ops | Settings | Office Profile Settings File |
| .opt | Settings | Options File |
| .opts | Settings | Linux Configuration Options File |
| .opx | Data | FlipAlbum Photo Album File |
| .opx | Data | OrgPlus Org Chart |
| .or2 | Data | Lotus Organizer 2 File |
| .or3 | Data | IBM Lotus Organizer 97 File |
| .or4 | Data | IBM Lotus Organizer 97 GS File |
| .or5 | Data | Lotus Organizer 5 File |
| .or6 | Data | Lotus Organizer 6 File |
| .orderedtest | Manufacturer | Visual Studio Ordered Test File |
| .orf | Raster Image | Olympus RAW File |
| .org | Data | Lotus Organizer File |
| .ori | Backup | Original File |
| .ort | Text | Rich Text Editor Document |
| .orx | Database | RadiantOne VDS Database Schema |
| .osax | Plugin | AppleScript Scripting Addition |
| .osp | Video | OpenShot Project File |
| .oss | Settings | Microsoft Outlook Saved Search File |
| .ost | Misc | Outlook Offline Folder File |
| .osx | Executable | PowerPC Executable File |
| .osz | Game | Osu! Beatmap File |
| .ota | Raster Image | OTA Bitmap Image |
| .otb | Raster Image | Nokia Over The Air Bitmap Image |
| .otc | Misc | OpenDocument Chart Template |
| .otf | Font | OpenType Font |
| .otf | Misc | OpenDocument Formula Template |
| .otf | Data | OpenTuft Tufting Design File |
| .otg | Vector Image | OpenDocument Graphic Template |
| .oth | Web | OpenDocument HTML Template |
| .oti | Raster Image | OpenDocument Image Template |
| .otl | Data | NoteTab Outline File |
| .otln | Data | Opal Outline File |
| .otm | Executable | Outlook Macro File |
| .otp | Data | OpenDocument Presentation Template |
| .ots | Audio | OtsAV Album File |
| .ots | Spreadsheet | OpenDocument Spreadsheet Template |
| .ott | Text | OpenDocument Text Document Template |
| .out | Data | Output File |
| .ov2 | GIS | TomTom Points of Interest Database |
| .ova | Data | Octava Musical Score |
| .ova | Data | Open Virtual Appliance |
| .ove | Audio | Overture Musical Score |
| .ovf | Data | Open Virtualization File |
| .ovl | Game | Rollercoaster Tycoon 3 Sprites File |
| .ovpn | Settings | OpenVPN Configuration File |
| .ovw | Audio | Logic Pro Overview File |
| .ovw | Audio | Cubase WAVE Overview File |
| .owc | Database | OutWit Catch Database |
| .owg | Misc | OutWit Gear Database |
| .owl | Manufacturer | OWL Source Code File |
| .owm | Misc | OutWit XML Mashup File |
| .oxt | Plugin | OpenOffice.org Extension |
| .oyx | Backup | Approach Alternate Database Index |
| .oz | Compressed | Opera Job Management Compressed File |