| Extension | 描述 | 檔案格式 |
|---|
| 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 | 網頁檔案 |
| ocx | ActiveX Control File | Developer Files |
| odg | OpenDocument Graphic File | 圖檔 |
| odp | OpenOffice Presentation File | 資料檔 |
| ods | OpenOffice Spreadsheet File | 資料檔 |
| odt | Open Office Document File | 文字檔 |
| oem | TextSetup OEM File | 設定檔 |
| oex | Opera Add-on File | 網頁檔案 |
| ofc | Open Financial Connectivity File | 資料檔 |
| ofx | Open Financial Exchange File | 資料檔 |
| ogg | Ogg Vorbis Compressed File | 聲音檔案 |
| ogm | Ogg Media File | 視頻檔案 |
| ogv | Video Container File | 視頻檔案 |
| old | Backup File | 備份檔案 |
| oma | OpenMG Music File | 聲音檔案 |
| oms | Order Maven File | Misc Files |
| one | OneNote Document File | 資料檔 |
| ont | Bible File | Misc Files |
| opd | Optical Path Difference File | 資料檔 |
| opf | Open Packaging Format File | Misc Files |
| opml | Outline Processor Markup Language File | Developer Files |
| ops | Office Profile Settings File | 設定檔 |
| opt | Option Files | 設定檔 |
| orc | Digital Orchestrator Pro/Plus Sound File | 聲音檔案 |
| org | Lotus Organizer File | 資料檔 |
| orig | Gen Original File | Misc Files |
| original | Component BBScomp File | 網頁檔案 |
| ost | Outlook Offline File | 資料檔 |
| out | Output File | 資料檔 |
| ov2 | TomTom Point Of Interest File | Misc Files |
| ove | Overture Score File | 聲音檔案 |
| ovl | Rollercoaster Tycoon Data File | 資料檔 |
| ovr | Microsoft Overlay File | 系統檔案 |
| oxt | Open Office Extension File | Misc Files |