File extensions start with character 'O'
| File Type | Category | File Format | |
|---|---|---|---|
| .oab | Microsoft Files | Offline Address Book | |
| .oam | Adobe Files | Adobe Edge Animate Widget File | |
| .oar | Compressed Files | OpenSimulator Archive | |
| .obb | Google Files | Android Opaque Binary Blob File | |
| .obd | Data Files | Office Binder Document | |
| .obj | Intel Files | Wavefront 3D Object File | |
| .obml | Opera Files | Opera Mini Saved Web Page | |
| .obw | Audio Files | Superior Drummer Sounds File | |
| .ocr | Text Files | FAXGrapper Fax Text File | |
| .ocx | Development Files | ActiveX Control | |
| .od | Data Files | Mascopt Graph Request File | |
| .odc | Configuration Files | Office Data Connection File | |
| .odf | Partially Downloaded Files | OpenDocument Formula | |
| .odif | Text Files | Open Document Interchange Format | |
| .odm | Text Files | OpenDocument Master Document | |
| .odp | Document Files | OpenDocument Presentation | |
| .ods | Spreadsheet Files | OpenDocument Spreadsheet | |
| .odt | Document Files | OpenDocument Text Document | |
| .odx | Data Files | BizTalk Server Orchestration File | |
| .oeaccount | Data Files | Windows Mail Account File | |
| .oeb | Backup Files | Outlook Express Backup Wizard File | |
| .ofc | Data Files | Open Financial Connectivity File | |
| .ofl | Text Files | Ots File List | |
| .ofm | Data Files | OmniForm Form | |
| .oft | Data Files | Outlook File Template | |
| .ofx | Data Files | Open Financial Exchange File | |
| .ogg | Audio Files | Ogg Vorbis Audio File | |
| .old | Backup Files | Backup File | |
| .olk | Data Files | Outlook Address Book File | |
| .olk14cate | Data Files | Outlook Category File | |
| .olk14cont | Data Files | Outlook Contact File | |
| .olk14even | Data Files | Outlook Calendar Event File | |
| .olk14grou | Data Files | Outlook Group File | |
| .olk14mail | Data Files | Outlook Mail Account File | |
| .olk14mess | Data Files | Outlook Message File | |
| .olk14msgs | Data Files | Outlook Email Message Data File | |
| .olk14note | Data Files | Outlook Note File | |
| .olk14sign | Data Files | Outlook Signature File | |
| .olk14task | Data Files | Outlook Task File | |
| .olt | Data Files | Orbit Downloads List | |
| .omcs | Data Files | Office Manager Access File | |
| .omg | Backup Files | OpenMG Audio File | |
| .omp | Data Files | Office Manager Document Archive | |
| .ond | Data Files | Lotus Notes Encapsulated Memo File | |
| .one | Data Files | OneNote Document | |
| .onepkg | Backup Files | Microsoft OneNote Package File | |
| .onetoc2 | Configuration Files | Microsoft OneNote Table of Contents File | |
| .oo3 | Data Files | OmniOutliner 3 File | |
| .opd | Page Layout Files | OmniPage Document | |
| .openbsd | Text Files | OpenBSD Readme File | |
| .opf | Project Files | Open Packaging Format File | |
| .ops | Configuration Files | Office Profile Settings File | |
| .opx | Data Files | OrgPlus Org Chart | |
| .or2 | Data Files | Lotus Organizer 2 File | |
| .or3 | Data Files | IBM Lotus Organizer 97 File | |
| .or4 | Data Files | IBM Lotus Organizer 97 GS File | |
| .or5 | Data Files | Lotus Organizer 5 File | |
| .or6 | Data Files | Lotus Organizer 6 File | |
| .org | Data Files | Lotus Organizer File | |
| .orig | Backup Files | Original File | |
| .ort | Text Files | Rich Text Editor Document | |
| .osx | Program Execution Files | PowerPC Executable File | |
| .otf | Font Files | OpenType Font | |
| .otl | Data Files | NoteTab Outline File | |
| .otln | Data Files | Opal Outline File | |
| .otp | Data Files | OpenDocument Presentation Template | |
| .otrkey | Digital Video Files | OnlineTvRecorder Encoded Video File | |
| .ots | Spreadsheet Files | OpenDocument Spreadsheet Template | |
| .ott | Text Files | OpenDocument Document Template | |
| .ott | Text Files | OpenOffice Text Template File | |
| .out | Data Files | Output File | |
| .ova | Data Files | Open Virtual Appliance | |
| .ovf | Data Files | Open Virtualization File | |
| .ovpn | Configuration Files | OpenVPN Configuration File | |
| .oxps | Page Layout Files | OpenXPS File | |
| .oz | Compressed Files | Opera Job Management Compressed File |