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