| .O1 | Open Source | Gegevensbestanden | Object File For GO321v1 Platform | Free Pascal | Open Source |
| .O3 | htmlland.net | Gegevensbestanden | Ozone Messenger Program Support Files | Ozone Messenger | htmlland.net |
| .OAB | Microsoft Corporation | Gegevensbestanden | Offline Address Book | Microsoft Outlook | Microsoft Corporation |
| .OAD | OA SISTEMI | Gegevensbestanden | Suite Notaro Document | Suite Notaro | OA SISTEMI |
| .OAF | Unknown Developer | Gegevensbestanden | ETH Oberon Applet | Unknown Software | Unknown Developer |
| .OAP | Samsung Electronics | Gegevensbestanden | Samsung Bada Signed Installation Package | Bada SDK | Samsung Electronics |
| .OB | Innoshock | Gegevensbestanden | Orbit Downloader Incomplete Download | Orbit Downloader | Innoshock |
| .OB1 | Unknown Developer | Gegevensbestanden | OzWin CompuServe E-mail/Forum Access Saved Outbox File | OzWin | Steve Sneed |
| .OBB | Google | Gegevensbestanden | Android Opaque Binary Blob File | Google Play Console | Google |
| .OBD | Microsoft Corporation | Gegevensbestanden | Office Binder Document | Microsoft Office | Microsoft Corporation |
| .OBS | Department of Wind Energy at the Technical University of Denmark, Denmark | Gegevensbestanden | WAsP Obstacle List Data | WAsP | Department of Wind Energy at the Technical University of Denmark, Denmark |
| .OBV | Mootools Software | Gegevensbestanden | 3D Photo Browser Thumbnail Cache | 3D Photo Browser | Mootools Software |
| .OC8 | EcoMerc | Gegevensbestanden | OrgCon Organization File | OrgCon | EcoMerc |
| .OC9 | EcoMerc | Gegevensbestanden | OrgCon Organization File | OrgCon | EcoMerc |
| .OCC | MathWorks | Gegevensbestanden | Oceanic DataPool File | The MathWorks MATLAB | MathWorks |
| .OCD | Ocad Inc. | Gegevensbestanden | OCAD Map File | OCAD | Ocad Inc. |
| .OCIMF | OCIMF | Gegevensbestanden | OCIMF HVPQ File | OCIMF SIRE | OCIMF |
| .OCN | OnTimeLine | Gegevensbestanden | OneCable.net - Explorer Measurement Data File | OneCable.net - Explorer | OnTimeLine |
| .OCP | Zylox Software, Inc. | Gegevensbestanden | Offline Commander Project File | Offline Commander | Zylox Software, Inc. |
| .OCS | Fujitsu Limited | Gegevensbestanden | eDoc DigiSign Digitally Signed Document | digiSeal Reader | Secrypt |
| .OCSMEET | Microsoft Corporation | Gegevensbestanden | Microsoft Lync Data | Microsoft Lync | Microsoft Corporation |
| .OCSP | Open Source | Gegevensbestanden | HAProxy OCSP Data File | HAProxy | Open Source |
| .OCV | Euresys | Gegevensbestanden | OCV Training Data | EasyOCV | Euresys |
| .OCW | Symantec Corporation | Gegevensbestanden | Symantec ESM Oracle Configuration Watch Template File | Symantec ESM | Symantec Corporation |
| .OD1 | Omnis Software | Gegevensbestanden | Omnis 5 Database | Omnis Studio | Omnis Software |
| .OD5 | Omnis Software | Gegevensbestanden | Omnis 5 Database | Omnis Studio | Omnis Software |
| .ODB | Various Developers | Gegevensbestanden | OpenDocument Database | Apache OpenOffice | Apache Software Foundation |
| .ODC | Microsoft Corporation | Gegevensbestanden | Office Data Connection File | Microsoft Excel | Microsoft Corporation |
| .ODCDATABASEFILE | Microsoft Corporation | Gegevensbestanden | Microsoft Office File | Microsoft Office | Microsoft Corporation |
| .ODCTABLECOLLECTIONFILE | Microsoft Corporation | Gegevensbestanden | Microsoft Office Component | Microsoft Office | Microsoft Corporation |
| .ODCTABLEFILE | Microsoft Corporation | Gegevensbestanden | Microsoft Office File | Microsoft Office | Microsoft Corporation |
| .ODF | Oracle Corporation | Gegevensbestanden | OpenDocument Formula | Apache OpenOffice | Apache Software Foundation |
| .ODG | Apache Software Foundation | Gegevensbestanden | OpenDocument Graphic File | Apache OpenOffice | Apache Software Foundation |
| .ODP | Oracle Corporation | Gegevensbestanden | OpenDocument Presentation | Apache OpenOffice | Apache Software Foundation |
| .ODR | Open Source | Gegevensbestanden | OpenIV Model Data | OpenIV | Open Source |
| .ODT | Oracle Corporation | Gegevensbestanden | OpenDocument Text Document | Apache OpenOffice | Apache Software Foundation |
| .ODV | Alfred-Wegener-Institut (AWI) | Gegevensbestanden | Ocean Data View Data | Ocean Data View | Alfred-Wegener-Institut (AWI) |
| .ODZ | IBM | Gegevensbestanden | IBM OpenDoc Editor Package | OpenDoc | IBM |
| .OEACCOUNT | Microsoft Corporation | Gegevensbestanden | Windows Mail Account File | Microsoft Mail | Microsoft Corporation |
| .OEC | Prometech Software, Inc. | Gegevensbestanden | OE-Cake File | OE-Cake | Prometech Software, Inc. |
| .OEM | Microsoft Corporation | Gegevensbestanden | Original Equipment Manufacturer Setup File | Microsoft Windows | Microsoft Corporation |
| .OFC | Microsoft Corporation | Gegevensbestanden | Open Financial Connectivity File | Reilly Technologies Moneydance | Infinite Kind |
| .OFD | Cypresslogic Systems Inc. | Gegevensbestanden | ObjectView Form Definition Data | ObjectView | Cypresslogic Systems Inc. |
| .OFM | Nuance | Gegevensbestanden | OmniForm Form | OmniForm | Nuance |
| .OFN | Microsoft Corporation | Gegevensbestanden | Systems Management Server Offer Creation Notification File | Microsoft Systems Management Server | Microsoft Corporation |
| .OFT | Microsoft Corporation | Gegevensbestanden | Outlook File Template | Microsoft Outlook | Microsoft Corporation |
| .OFX | Intuit, Inc. | Gegevensbestanden | Open Financial Exchange File | Intuit Quicken | Quicken |
| .OGF2 | Unknown Developer | Gegevensbestanden | MapViewGPS Map Coordinates File | Unknown Software | Unknown Developer |
| .OGM | VideoLAN | Gegevensbestanden | Ogg Media File | VLC Media Player | VideoLAN |
| .OGS | OriginLab | Gegevensbestanden | Origin LabTalk Script File | Origin | OriginLab |
| .OH | Efka-Soft | Gegevensbestanden | DrillAssistant List Data File | DrillAssistant | Efka-Soft |
| .OH4 | Efka-Soft | Gegevensbestanden | Overhoor List File | Overhoor | Efka-Soft |
| .OHP | Microsoft Corporation | Gegevensbestanden | MS-DOS 7 File | MS-DOS | Microsoft Corporation |
| .OHS | RAD Game Tools | Gegevensbestanden | Bink Outbound History File | RAD Video Tools | RAD Game Tools |
| .OHT | Oracle Corporation | Gegevensbestanden | Oracle Help For Java Links Data File | Oracle Help For Java | Oracle Corporation |
| .OHW | Efka-Soft | Gegevensbestanden | DrillAssistant List File | DrillAssistant | Efka-Soft |
| .OID | Hewlett-Packard | Gegevensbestanden | HP ProCurve Manager Network Device File | HP ProCurve Manager | Hewlett-Packard |
| .OK | Robelle | Gegevensbestanden | QEDIT Data File | QEDIT | Robelle |
| .OKD | Lenovo | Gegevensbestanden | Lenovo OneKey Recovery Data | Lenovo OneKey Recovery | Lenovo |
| .OKX | Mike Mardesich | Gegevensbestanden | OKScript File | OKScript | Mike Mardesich |
| .OLB | Microsoft Corporation | Gegevensbestanden | OLE Object Library | Microsoft Excel | Microsoft Corporation |
| .OLE | Microsoft Corporation | Gegevensbestanden | Microsoft Object Linking And Embedding Data | Microsoft Windows | Microsoft Corporation |
| .OLK | Microsoft Corporation | Gegevensbestanden | Outlook Address Book File | Microsoft Word | Microsoft Corporation |
| .OLL | inData | Gegevensbestanden | TrialDirector Object Load List | TrialDirector | inData |
| .OLM | Microsoft Corporation | Gegevensbestanden | Mac Outlook Database File | Microsoft Outlook 2016 | Microsoft Corporation |
| .OLR | PerkinElmer | Gegevensbestanden | Openlab RAW Image File | Openlab | PerkinElmer |
| .OLS | Microsoft Corporation | Gegevensbestanden | Microsoft Office List Shortcut | Microsoft Office | Microsoft Corporation |
| .OLT | Innoshock | Gegevensbestanden | Orbit Downloads List | Orbit Downloader | Innoshock |
| .OMCS | Krekeler | Gegevensbestanden | Office Manager Access File | Krekeler Office Manager Pro | Krekeler |
| .OMP | MatchWare | Gegevensbestanden | OpenMind Windows Document | MindView for Windows | MatchWare |
| .OMR | Orbit GeoSpatial Technologies | Gegevensbestanden | Orbit Mobile Mapping Resource Fle | ORBIT GIS | Orbit GeoSpatial Technologies |
| .OMS | Hewlett-Packard | Gegevensbestanden | HP Printer Substrate Presets Package File | HP Embedded Web Server | Hewlett-Packard |
| .OND | IBM | Gegevensbestanden | Lotus Notes Encapsulated Memo File | IBM Lotus Notes | IBM |
| .ONE | Microsoft Corporation | Gegevensbestanden | OneNote Document | Microsoft OneNote | Microsoft Corporation |
| .ONEPASSWORD-LICENSE | AgileBits, Inc. | Gegevensbestanden | 1Password License | 1Password | AgileBits, Inc. |
| .ONLINE | Apple | Gegevensbestanden | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .ONT | Open Source | Gegevensbestanden | TheWord Bible Text Module | theWord | Costas stergiou |
| .ONTX | Open Source | Gegevensbestanden | TheWord Encrypted Bible Text Module | theWord | Costas stergiou |
| .ONW | Omron Corporation | Gegevensbestanden | Omron HMI NTST Data | Omron NTST | Omron Corporation |
| .OO3 | The Omni Group | Gegevensbestanden | OmniOutliner 3 File | OmniOutliner | The Omni Group |
| .OOA | Sublime HQ Pty Ltd | Gegevensbestanden | OOA Interchange Format Data | Notepad++ | Don Ho |
| .OOT | Apache HTTP Project Management Committee | Gegevensbestanden | Apache OpenOffice Text Document | Apache OpenOffice | Apache Software Foundation |
| .OP | Open Source | Gegevensbestanden | OpenPAT Report | OpenPAT | Open Source |
| .OPD | Nuance | Gegevensbestanden | OmniPage Document | OmniPage | Nuance |
| .OPF | E-Book Systems | Gegevensbestanden | FlipViewer FlipBook File | FlipViewer | E-Book Systems |
| .OPI | Adlib Tracker II | Gegevensbestanden | Adlib OCR Processing Information File | Adlib Express | Adlib Tracker II |
| .OPJ | Cadence Design Systems | Gegevensbestanden | OrCAD Project File | OrCAD PCB Designer | Cadence Design Systems |
| .OPK | OriginLab | Gegevensbestanden | Origin Pack File | Origin | OriginLab |
| .OPM | Quadrox, Inc. | Gegevensbestanden | Quadrox Digital CCTV System Components | Quadrox Digital CCTV System Components | Quadrox, Inc. |
| .OPO | Psion PLC | Gegevensbestanden | Psion OPL Object File | Unknown Software | Unknown Developer |
| .OPP | Keronsoft | Gegevensbestanden | iPhotoDVD File | iPhotoDVD | Keronsoft |
| .OPW | Insperity Business Services | Gegevensbestanden | OrgPlus For Windows Organization Chart | Insperity OrgPlus | Insperity Business Services |
| .OPX | Insperity Business Services | Gegevensbestanden | OrgPlus Organizational Chart | Microsoft Office | Microsoft Corporation |
| .OR | Apple | Gegevensbestanden | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .OR2 | IBM | Gegevensbestanden | Lotus Organizer 2 File | IBM Lotus Organizer | IBM |
| .OR3 | IBM | Gegevensbestanden | IBM Lotus Organizer 97 File | IBM Lotus Organizer | IBM |
| .OR4 | IBM | Gegevensbestanden | IBM Lotus Organizer 97 GS File | IBM Lotus Forms Viewer | IBM |
| .OR5 | IBM | Gegevensbestanden | Lotus Organizer 5 File | IBM Lotus Organizer | IBM |
| .OR6 | IBM | Gegevensbestanden | Lotus Organizer Data | IBM Lotus Organizer | IBM |
| .ORC | Voyetra Turtle Beach | Gegevensbestanden | Voyetra Digital Orchestrator File | Digital Orchestrator | Voyetra Turtle Beach |
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