| Extension | Description | File Type |
|---|
| o | Free Pascal Object File | Developer Files |
| ob! | Orbit Downloader Incomplete Download | Misc Files |
| obj | 3D Objects File | 3D Files |
| obml15 | Opera Mini Saved Web Page | Web Files |
| ocx | ActiveX Control File | Developer Files |
| odg | OpenDocument Graphic File | Graphic Files |
| odp | OpenOffice Presentation File | Data Files |
| ods | OpenOffice Spreadsheet File | Data Files |
| odt | Open Office Document File | Text Files |
| oem | TextSetup OEM File | Settings Files |
| oex | Opera Add-on File | Web Files |
| ofc | Open Financial Connectivity File | Data Files |
| ofx | Open Financial Exchange File | Data Files |
| ogg | Ogg Vorbis Compressed File | Audio Files |
| ogm | Ogg Media File | Video Files |
| ogv | Video Container File | Video Files |
| old | Backup File | Backup Files |
| oma | OpenMG Music File | Audio Files |
| oms | Order Maven File | Misc Files |
| one | OneNote Document File | Data Files |
| ont | Bible File | Misc Files |
| opd | Optical Path Difference File | Data Files |
| opf | Open Packaging Format File | Misc Files |
| opml | Outline Processor Markup Language File | Developer Files |
| ops | Office Profile Settings File | Settings Files |
| opt | Option Files | Settings Files |
| orc | Digital Orchestrator Pro/Plus Sound File | Audio Files |
| org | Lotus Organizer File | Data Files |
| orig | Gen Original File | Misc Files |
| original | Component BBScomp File | Web Files |
| ost | Outlook Offline File | Data Files |
| out | Output File | Data Files |
| ov2 | TomTom Point Of Interest File | Misc Files |
| ove | Overture Score File | Audio Files |
| ovl | Rollercoaster Tycoon Data File | Data Files |
| ovr | Microsoft Overlay File | System Files |
| oxt | Open Office Extension File | Misc Files |