| .O | Microsoft Corporation | Udviklerfiler | Compiled Object File | Microsoft Visual Studio | Microsoft Corporation |
| .OAB | Microsoft Corporation | Datafiler | Offline Address Book | Microsoft Outlook | Microsoft Corporation |
| .OBD | Microsoft Corporation | Datafiler | Office Binder Document | Microsoft Office | Microsoft Corporation |
| .OBI | Microsoft Corporation | Indstillingsfiler | Outlook RSS Subscription File | Microsoft Outlook 2016 | Microsoft Corporation |
| .OBT | Open Source | Indstillingsfiler | Openbox Theme File | Microsoft Binder | Microsoft Corporation |
| .OCA | Microsoft Corporation | Udviklerfiler | Custom Control Library Type File | Microsoft Visual Basic | Microsoft Corporation |
| .OCR | URUWORKS | Ikke Almindelige Filer | Subtitle Workshop OCR Script | Microsoft Notepad | Microsoft Corporation |
| .OCSMEET | Microsoft Corporation | Datafiler | Microsoft Lync Data | Microsoft Lync | Microsoft Corporation |
| .OCX | Microsoft Corporation | Udviklerfiler | ActiveX Control | Microsoft Visual Studio | Microsoft Corporation |
| .OC_ | Microsoft Corporation | Komprimerede filer | Compressed ActiveX Control File | Microsoft Windows | Microsoft Corporation |
| .ODC | Microsoft Corporation | Indstillingsfiler | Office Data Connection File | Microsoft Excel | Microsoft Corporation |
| .ODCDATABASEFILE | Microsoft Corporation | Datafiler | Microsoft Office File | Microsoft Office | Microsoft Corporation |
| .ODCTABLECOLLECTIONFILE | Microsoft Corporation | Datafiler | Microsoft Office Component | Microsoft Office | Microsoft Corporation |
| .ODCTABLEFILE | Microsoft Corporation | Datafiler | Microsoft Office File | Microsoft Office | Microsoft Corporation |
| .ODF | Oracle Corporation | Ikke Almindelige Filer | OpenDocument Formula | Microsoft Notepad | Microsoft Corporation |
| .ODL | Microsoft Corporation | Udviklerfiler | Object Description Language File | Microsoft Visual Studio | Microsoft Corporation |
| .ODP | Oracle Corporation | Datafiler | OpenDocument Presentation | Microsoft PowerPoint | Microsoft Corporation |
| .ODS | Oracle Corporation | Regnearksfiler | OpenDocument Spreadsheet | Microsoft Excel | Microsoft Corporation |
| .ODT | Oracle Corporation | Tekstfiler | OpenDocument Text Document | Microsoft Word | Microsoft Corporation |
| .OEACCOUNT | Microsoft Corporation | Datafiler | Windows Mail Account File | Microsoft Mail | Microsoft Corporation |
| .OEM | Microsoft Corporation | Indstillingsfiler | Original Equipment Manufacturer Setup File | Microsoft Windows | Microsoft Corporation |
| .OFC | Microsoft Corporation | Datafiler | Open Financial Connectivity File | Microsoft Money | Microsoft Corporation |
| .OFFICEUI | Microsoft Corporation | Indstillingsfiler | Microsoft Office UI Customization File | Microsoft Office | Microsoft Corporation |
| .OFN | Microsoft Corporation | Datafiler | Systems Management Server Offer Creation Notification File | Microsoft Systems Management Server | Microsoft Corporation |
| .OFT | Microsoft Corporation | Datafiler | Outlook File Template | Microsoft Outlook | Microsoft Corporation |
| .OFX | Intuit, Inc. | Datafiler | Open Financial Exchange File | Microsoft Money | Microsoft Corporation |
| .OGX | Xiph.Org | Videofiler | Ogg Vorbis Multiplexed Media File | Microsoft Windows Media Player | Microsoft Corporation |
| .OHP | Microsoft Corporation | Datafiler | MS-DOS 7 File | MS-DOS | Microsoft Corporation |
| .OLB | Microsoft Corporation | Ikke Almindelige Filer | OLE Object Library | Microsoft Excel | Microsoft Corporation |
| .OLE | Microsoft Corporation | Datafiler | Microsoft Object Linking And Embedding Data | Microsoft Windows | Microsoft Corporation |
| .OLK | Microsoft Corporation | Datafiler | Outlook Address Book File | Microsoft Word | Microsoft Corporation |
| .OLM | Microsoft Corporation | Datafiler | Mac Outlook Database File | Microsoft Outlook 2016 | Microsoft Corporation |
| .OLS | Microsoft Corporation | Datafiler | Microsoft Office List Shortcut | Microsoft Office | Microsoft Corporation |
| .OMP | MatchWare | Sideopsætningsfiler | OpenMind Windows Document | Microsoft Access | Microsoft Corporation |
| .ONE | Microsoft Corporation | Datafiler | OneNote Document | Microsoft OneNote | Microsoft Corporation |
| .ONEBIN | Microsoft Corporation | Midlertidige Filer | OneNote Data | Microsoft OneNote | Microsoft Corporation |
| .ONECACHE | Microsoft Corporation | Midlertidige Filer | Microsoft OneNote Cache File | Microsoft OneNote | Microsoft Corporation |
| .ONEPKG | Microsoft Corporation | Sikkerhedskopifiler | Microsoft OneNote Package File | Microsoft OneNote | Microsoft Corporation |
| .ONETOC | Microsoft Corporation | Indstillingsfiler | Microsoft OneNote Table Of Contents | Microsoft OneNote | Microsoft Corporation |
| .ONETOC2 | Microsoft Corporation | Indstillingsfiler | Microsoft OneNote Table Of Contents File | Microsoft OneNote | Microsoft Corporation |
| .OPS | Microsoft Corporation | Indstillingsfiler | Office Profile Settings File | Microsoft Office | Microsoft Corporation |
| .OPT | LexisNexis | Databasefiler | Concordance Database File | Microsoft Visual Studio | Microsoft Corporation |
| .OPUS | Open Source | Lydfiler | Opus Audio File | Microsoft Windows Media Player | Microsoft Corporation |
| .OPX | Insperity Business Services | Datafiler | OrgPlus Organizational Chart | Microsoft Office | Microsoft Corporation |
| .OQY | Microsoft Corporation | Databasefiler | Excel OLAP Query File | Microsoft Excel | Microsoft Corporation |
| .ORDEREDTEST | Microsoft Corporation | Udviklerfiler | Visual Studio Ordered Test File | Microsoft Visual Studio | Microsoft Corporation |
| .ORF | Olympus Corporation | Kamera Ubehandlede Filer | Olympus RAW File | ACD Systems Canvas | Microsoft Corporation |
| .ORP | PV & Associates | Datafiler | WinSLAM File | Microsoft Translator | Microsoft Corporation |
| .OSC | OpenStreetMap | GIS-filer | OpenStreetMap Change File | Microsoft Windows | Microsoft Corporation |
| .OSD | Microsoft Corporation | Datafiler | Open Software Description Data | Microsoft Application Virtualization | Microsoft Corporation |
| .OSDX | Microsoft Corporation | Indstillingsfiler | Search Connector Description File | Microsoft Windows | Microsoft Corporation |
| .OSS | Microsoft Corporation | Indstillingsfiler | Microsoft Outlook Saved Search File | Microsoft Outlook | Microsoft Corporation |
| .OST | Microsoft Corporation | Ikke Almindelige Filer | Outlook Offline Data File | Microsoft Outlook 2016 | Microsoft Corporation |
| .OTM | Microsoft Corporation | Eksekverbare filer | Outlook Macro File | Microsoft Word | Microsoft Corporation |
| .OTP | Oracle Corporation | Datafiler | OpenDocument Presentation | Microsoft PowerPoint | Microsoft Corporation |
| .OUT | Open Source | Eksekverbare filer | Compiled Executable File | Microsoft Notepad | Microsoft Corporation |
| .OV3 | Microsoft Corporation | Systemfiler | DOS Overlay File | MS-DOS | Microsoft Corporation |
| .OXPS | Microsoft Corporation | Sideopsætningsfiler | OpenXPS File | Microsoft XPS Viewer | Microsoft Corporation |
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