O

File extensions starting with the letter O...

  .O    Object File (various)
  .O$$  Output File (temporary usually)
  .O01  Typhoon Voice File
  .OAB  MS Outlook Address Book
  .OAZ  OAZ Fax File  
  .OBD  MS Office Binder
  .OBE  MS Out Of Box Experience
  .OBJ  Relocatable Object Code (various)
        Lightwave Object
  .OBZ  MS Office Binder Wizard
  .OCA  Control Typelib Cache
  .OCX  Object Linking and Embedding (OLE) Control Extension
  .OD1  Omnis5 Database File (also .OD2, .OD3, etc.)
  .ODE  MS Office Object Data Embedding File
  .ODF  Open Document Interchange
        BattleZone Cartographers Guild File
  .ODL  Visual C++ Type Library Source
  .ODS  MS Outlook Express Mailbox
  .OFC  Open Financial Connectivity File

  .OFF  Vector Graphic (Object File Format)
        3D Mesh Object File Format
  .OFM  Adobe Font
  .OFN  MS Office FileNew File
  .OFT  MS Outlook Item Template
  .OFX  Open Financial Exchange File
  .OGG  Ogg Vorbis Codec Compressed WAV File
        Origin File
  .OGS  Origin File
  .OKT  Oktalyzer Tracker Module
  .OLB  Object Library
  .OLD  Old Version (sometimes used as a backup indicator)
  .OLE  Object Linking and Embedding (OLE) Object
  .OLI  Olivetti Text File
  .OMF  Image File
  .OND  Lotus Notes-related File (unverified)
  .ONX  Onyx Graphics Postershop File
  .OPC  Beta 44 Job File
  .OPJ  OrCad Project File
        Origin File
  .OPL  Psion Organiser Programming Language Source File
  .OPO  OPL Output Executable File
  .OPT  QEMM Optimize
        Options
        MS Developer Studio File
  .OPX  MS Orgchart Organization Diagram
        OPL Extension DLL
  .OQY  MS Excel OLAP Query File
  .OR2  Lotus Organiser 2 File
  .OR3  Lotus Organiser 97 File
  .ORA  Oracle 7 Configuration
  .ORC  MIDI File
        Oracle 7 Script
  .ORG  Lotus Organiser File
        Origin File
  .ORI  Original
  .OSS  MS Office Saved Search
  .OST  MS Exchange/Outlook Offline File
  .OTL  Super NoteTab Template File
  .OTP  Origin File
  .OUT  Outlines or Output File (various)
  .OV1  Program File - Overlay
  .OV2  Program File - Overlay
  .OV3  Program File - Overlay
  .OVE  Cakewalk Overture Score
  .OVL  Program File - Overlay
  .OVR  Program File - Overlay
  .OZM  Sharp Organizer Memo Bank
  .OZP  Sharp Organizer Telephone Bank