| Extension |
File Type |
File Description |
| .o | Developer | Compiled Object File |
| .oab | Data | Offline Address Book |
| .ob! | Misc | Orbit Downloader Incomplete Download |
| .obj | 3D Image | 3D Object File |
| .obj | Data | Relocatable Object Code |
| .obs | Executable | ObjectScript Script File |
| .oca | Developer | Custom Control Library Type File |
| .ocr | Text | FAXGrapper Fax Text File |
| .ocx | Developer | ActiveX Control |
| .odb | Data | ABAQUS Database |
| .odc | Image | OpenDocument Chart |
| .odf | Misc | OpenDocument Formula |
| .odf | Data | Open Document File |
| .odg | Image | OpenDocument Graphic |
| .odi | Image | OpenDocument Image |
| .odif | Image | Open Document Interchange Format |
| .odm | Text | OpenDocument Master Document |
| .odm | Audio | OverDrive Media File |
| .odp | Data | OpenDocument Presentation |
| .ods | Misc | Outlook Express 5 Mailbox |
| .ods | Data | OpenDocument Spreadsheet |
| .odt | Text | OpenDocument Text Document |
| .odx | Data | BizTalk Server Orchestration File |
| .oeb | Backup | Outlook Express Backup Wizard File |
| .ofc | Data | Open Financial Connectivity File |
| .off | Image | Object File Format |
| .ofl | Text | Ots File List |
| .ofm | Data | OmniForm Form |
| .oft | Text | Outlook File Template |
| .ofx | Data | Open Financial Exchange File |
| .ogg | Audio | Ogg Vorbis Compressed Audio File |
| .ogm | Video | Ogg Media File |
| .ognc | Web | Dynamic Web Page |
| .okt | Audio | Oktalyzer Module |
| .ola | Page Layout | Online Access File |
| .olb | Misc | OLE Object Library |
| .old | Backup | Backup File |
| .oma | Audio | Sony OpenMG Music File |
| .omf | Audio | Open Media Framework File |
| .omf | Raster Image | OMF Interchange Image File |
| .omg | Audio | OpenMG Audio File |
| .omo | Developer | OMake Object File |
| .omx | Audio | OtsAV Media Library Information File |
| .ond | Data | Lotus Notes Encapsulated Memo File |
| .one | Data | OneNote Document |
| .oo3 | Data | OmniOutliner 3 File |
| .opd | Page Layout | OmniPage Document |
| .opf | Image | FlipAlbum File |
| .opf | Data | FlipViewer FlipBook File |
| .opml | Text | OPML File |
| .ops | Settings | Office Profile Settings File |
| .opt | Settings | Options File |
| .opx | Data | OrgPlus Org Chart |
| .opx | Data | FlipAlbum Photo Album File |
| .or4 | Data | Lotus Organizer 4 File |
| .orf | Image | Olympus RAW File |
| .org | Data | Lotus Organizer File |
| .ori | Backup | Original File |
| .ost | Misc | Outlook Offline Folder File |
| .ota | Image | OTA Bitmap |
| .otb | Image | Nokia Over The Air Bitmap |
| .otc | Image | OpenDocument Chart Template |
| .otf | Font | OpenType Font |
| .otf | Misc | OpenDocument Formula Template |
| .otf | Data | OpenTuft Tufting Design File |
| .otg | Image | OpenDocument Graphic Template |
| .oth | Web | OpenDocument HTML Template |
| .oti | Image | OpenDocument Image Template |
| .otln | Data | Opal Outline File |
| .otp | Misc | OpenDocument Presentation Template |
| .ots | Data | OpenDocument Spreadsheet Template |
| .ots | Audio | OtsAV Album File |
| .ott | Text | OpenDocument Text Document Template |
| .ov2 | Data | TomTom Points of Interest Database |
| .ova | Data | Octava Musical Score |
| .ove | Misc | Overture Musical Score |
| .ovl | Data | Rollercoaster Tycoon 3 Sprites File |
| .ovpn | Settings | OpenVPN Configuration File |
| .ovw | Image | Cubase WAVE File Overview |
| .owc | Game | OutWit Catch Database |
| .owg | Misc | OutWit Gear Database |
| .owm | Misc | OutWit XML Mashup File |
| .oyx | Backup | Approach Alternate Database Index |
