letter O extensions

.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
.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
.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
.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
.ORI Original
.OSS MS Office Saved Search
.OST MS Exchange/Outlook Offline File
.OTL Super NoteTab Template 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