List of Data File Formats Starting with the Letter "O"
| .o | Object file (unix - Atari - GCC) |
| .o$$ | Outfile (Sprint) |
| .oaz | Fax (NetFax Manager) |
| .ob | Object cut/paste file (IBM LinkWay) |
| .obd | MS Office Binder |
| .obj | Object code (Intel Recolatable Object Module) |
| .obr | Object browser data file (Borland C++) |
| .obs | Script (ObjectScript) |
| .obv | Visual interface (ObjectScript) |
| .oca | Control Typelib Cache |
| .ocf | Object Craft File (Object Craft) |
| .ocm | AOL Advertising Control files |
| Internet Odyssey 2 Update | |
| .ocp | Advanced Art Studio |
| Offline Commander Project file | |
| .ocr | Incoming fax transcribed to text (FAXGrapper) |
| .oct | Radiance Octree Format |
| .ocx | OLE ActiveX custom control |
| .odf | Open Document Interchange |
| BattleZone Cartographers Guild file | |
| Star Trek Armada Ship/Structure Infomation | |
| .odg | OpenDocument Graphic file |
| .odl | Type library source (Visual C++) |
| .odp | OpenOffice Presentation file |
| .ods | OpenOffice Spreadsheet file |
| .odt | OpenOffice OpenDocument text document |
| .oeb | Outlook Express Backup Wizard |
| .oem | TextSetup OEM file |
| .ofc | Open Financial Connectivity file |
| .ofd | Form definition (ObjectView) |
| .off | Object File Format vector graphics |
| .ofm | Adobe font |
| .oft | MS Outlook Item Template |
| .ofx | Olicom Fax |
| Open Financial Exchange file | |
| .ogg | Ogg Vorbis Codec Compressed WAV file |
| .ogm | Ogg Vorbis Compressed Video file |
| .ogv | Video Container file |
| .okt | Music (Oktalizer) |
| .olb | Object library (VAX) |
| .old | Backup file |
| .ole | Object Linking and Embedding Object |
| .oli | Text file (Olivetti) |
| .oma | OpenMG Music file |
| .omf | Open Media file |
| .omg | OpenMG Jukebox |
| .oms | Briggs Softworks Order Maven |
| Macintosh MP3 Music Format | |
| Omega Downloader Configuration file | |
| .ond | Lotus Notes-related file |
| .one | OneNote Document File |
| .ont | Bible file |
| .oom | Swap file (Shroom) |
| .opd | Omnipage file |
| .opf | Flip Album file |
| Open Packaging Format file | |
| .opl | Psion Organiser Programming Language Source file |
| .opn | Active options (Exact) |
| .ops | Microsoft Office profile settings file |
| .opt | Optimize support file (QEMM) |
| .opw | Organization chart (Org Plus for Windows) |
| .opx | Inactive options (Exact) |
| .or2 | Lotus Organizer 2 file |
| .or3 | Lotus Organizer 97 file |
| .or4 | Lotus Organizer file |
| .or5 | Lotus Organizer file |
| .ora | Parameter file (Oracle) |
| .org | Calendar file (Lotus Organizer) |
| .osd | Open Software Description file |
| .oss | MS Office Saved Search |
| .ost | Microsoft Outlook Offline file |
| .otf | Open Type Format |
| .otl | Outline font description (Z-Soft Type Foundry) |
| .otx | Text file (Olivetti Olitext Plus) |
| .out | Output file |
| .ov1 | Overlay file (part of program to be loaded when needed) |
| .ov2 | Overlay file (part of program to be loaded when needed) |
| .ovd | Datafile (ObjectVision) |
| .ovl | Overlay file (part of program to be loaded when needed) |
| .ovr | Overlay file (part of program to be loaded when needed) |
| .ovw | Cool Edit Pro Overviewfile |
| Cubase .WAV File Image | |
| DIANA Overview file | |
| .ows | Web Studio 2 Project file |
| .oxt | Open Office Extension file |
