.??_ Files ending in "_" are often compressed install files of the same
name (e.g., .EX_ expands to .EXE, .DL_ expands to .DLL, etc.)
.O Object File (various)
.O$$ Output File (temporary usually)
.O01 Typhoon Voice File
.OAB Microsoft Outlook Address Book  [Buy Office XP at Shop.Microsoft]
.OAZ NetFax Manager OAZ Fax File [XnView]
.OB IMB LinkWay Object File
.OB1 OzWin CompuServe E-mail/Forum Access Saved Outbox File
.OBD Microsoft Office Binder [Buy Office XP at Shop.Microsoft]
.OBE Microsoft Out Of Box Experience
.OBJ Relocatable Object Code (various)
Lightwave Object
.OBN Oberon-2 Module Source File
.OBPACK ObjectBar Object (customizes the Windows Taskbar)
.OBTHEME ObjectBar Theme File (customizes the Windows Taskbar)
.OBR Borland C++ Object Browser Data File
.OBS ObjectScript Script
OzWin CompuServe E-mail/Forum Access Saved Outbox File
.OBT Microsoft Office Binder Template [Buy Office XP at Shop.Microsoft]
.OBV ObjectScript Visual Interface
.OBZ Microsoft Office Binder Wizard [Buy Office XP at Shop.Microsoft]
.OCA Control Typelib Cache
.OCC DB/TextWorks Database Terms and Words (indexed by .BTX file)
.OCF Object Craft File
OM2Cipher File
.OCM AOL Advertising Control Files (advert.ocm & admin.ocm)
Internet Odyssey 2 Update
.OCP Advanced Art Studio [XnView]
Offline Commander Project File
.OCR FAXGrapper FAX Text File
.OCT Radiance Octree Format
.OCX Object Linking and Embedding (OLE) Control Extension (ActiveX)
.OD1 Omnis5 Database File (also .OD2, .OD3, etc.)
.ODB ArcView Object Database ASCII File (ODB format)
.ODC Microsoft Office Data Connection
.ODCCUBEFILE Microsoft Office Data Connection
.ODCDATABASEFILE Microsoft Office Data Connection
.ODCNEWFILE Microsoft Office Data Connection
.ODCTABLEFILE Microsoft Office Data Connection
.ODE Microsoft Office Object Data Embedding File
[Buy Office XP at Shop.Microsoft]
.ODF Open Document Interchange
BattleZone Cartographers Guild File
Star Trek Armada Ship/Structure Infomation
.ODL Visual C++ Type Library Source
.ODP Oedipus (genealogy)
.ODS Microsoft Outlook Express Mailbox [Buy Office XP at Shop.Microsoft]
.OEM Text Listing of Supported OEM Devices for WinNT (TXTSETUP.OEM)
OEM Data Used During Device Install (various)
.OF LessDATA Video System OneFile File (contains both video and/or audio)
.OFC Open Financial Connectivity File
.OFD ObjectView Form Defintion
.OFF Vector Graphic (Object File Format)
3D Mesh Object File Format
.OFM Adobe Font
OmniForm Form File
Embroidery Design File [possibly read with Melco Sizer Software]
.OFN Microsoft Office FileNew File [Buy Office XP at Shop.Microsoft]
.OFT Microsoft Outlook Item Template [Buy Office XP at Shop.Microsoft]
.OFX Open Financial Exchange File (used by programs like Quicken)
Olicom Fax [XnView]
.OGG Ogg Vorbis Codec Compressed WAV File [use Winamp]
Origin File
.OGM Ogg Vorbis Compressed Video File
.OGS Origin File
.OHI Delphi Configuration File (ASCII text file)
.OHL Delphi Configuration File (ASCII text file)
.OIF QuickBase Web-sharable Database Interchange File
.OIL Open Image Library Format [XnView]
.OKS OKScript File (Windows scripting tool)
.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
.OLK Microsoft Outlook Address Book
.OM Omicron Compiler File
.OMF Open Media Format File
(Developed by Avid and can contain images, video, etc.)
.OMG OpenMG Jukebox (Sony's proprietary ATRAC3 codec)
.OMN Omnipotent, Inc. Collection File
(ASCII text file relating to explosives, etc.)
.OMO Oracle Media Objects File
.OMS Omega Downloader Configuration File (text)
Briggs Softworks Order Maven (Order form creation utility)
Macintosh MP3 Music Format
.OND Lotus Notes-related File
(small Notes database with one document; OND=One Note Database)
.ONX Onyx Graphics Postershop File
.OOM Shroom Swap File
.OP2 Outpost 2 Saved Game [Supporting Site]
.OPC Beta 44 Job File
Omron OPC Client File (Factory Automation)
Microsoft Upgrade Control File (ASCII text file)
.OPD OmniPage12 Document
.OPF Flip Album File
.OPJ OrCad Project File
Origin File
.OPL Psion Organiser Programming Language Source File
.OPM Quadrox Digital CCTV System Components (opshellext.dll)
.OPN Exact Active Options
.OPO OPL Output Executable File
.OPS Microsoft Office Profile Settings File [Viewer]
.OPT QEMM Optimize
Options
Microsoft Developer Studio File
Segue SilkTest Option Set File
(previously QA Partner or QualityWorks - ASCII text file)
.OPX Microsoft Orgchart Organization Diagram
Exact Inactive Options
OPL Extension DLL
Flip Album File
.OQY Microsoft Excel OLAP Query File [Buy Office XP at Shop.Microsoft]
.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
.OSC Remote Installation Services Client Installation Wizard File
(OSChooser Markup Language)
.OSD Open Software Description File
(describe software components, versions, underlying structures, and dependence)
.OSF ICZ OriSAFE (file encryption and signing through Microsoft CryptoAPI)
.OSG OSG321 Software Application Data File
.OSS Microsoft Office Saved Search [Buy Office XP at Shop.Microsoft]
.OST Microsoft Exchange/Outlook Offline File
.OTB Nokia Phone Graphics Editor [XnView]
On-the-air Bitmap
.OTF Open Type Font Format
.OTHERFOLDER OTHERFOLDER Powertoy File
.OTL NoteTab Outline
Z-Soft Type Foundry Font
.OTP Origin File
Top Secret Crypto Gold File
.OTS OtsJuke Sound File
.OTX Olivetti Olitext Plus File
.OU! OzWin CompuServe E-mail/Forum Access Outbound Messages Online
.OUT Outlines or Output File (various)
OzWin CompuServe E-mail/Forum Access Outbound Messages
.OV1 Program File - Overlay
.OV2 Program File - Overlay
.OV3 Program File - Overlay
.OVA Sierra Web Studio Graphic
.OVD ObjectVision Datafile
.OVE Cakewalk Overture Score
.OVL Program File - Overlay
.OVR Program File - Overlay
.OVW Cool Edit Pro Overview File
DIANA Overview File
Cubase .WAV File Image
.OWD OpenWire Project
(Delphi, C++ Builder and Kylix graphical design library)
.OWS Web Studio 2 Project File
.OX Ox Object-Oriented Matrix Programming Language File
.OXL OMEGA Product Suite File (Predictive data mining)
.OXO Ox Object-Oriented Matrix Programming Language File
.OZM Sharp Organizer Memo Bank
.OZP Sharp Organizer Telephone Bank
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