Alphabetically: O
Extension File Description File Type
.o
Compiled Object File
Developer
.oab
Offline Address Book
Data
.oar
OpenSimulator Archive
Compressed
.ob!
Orbit Downloader Incomplete Download
Misc
.obi
Outlook RSS Subscription File
Settings
.obj
3D Object File
3D Image
.obml
Opera Mini Saved Web Page
Web
.obp
Bryce Object File
3D Image
.obs
ObjectScript Script File
Executable
.obz
Compressed 3D Object File
3D Image
.oca
Custom Control Library Type File
Developer
.oce
Open Catalog Extension Connection File
Settings
.ocr
FAXGrapper Fax Text File
Text
.ocx
ActiveX Control
Developer
.od
Mascopt Graph Request File
Data
.odb
OpenOffice.org Base Database
Database
.odc
Office Data Connection File
Settings
.odf
OpenDocument Formula
Misc
.odg
OpenDocument Graphic File
Vector Image
.odi
OpenDocument Image
Raster Image
.odif
Open Document Interchange Format
Text
.odm
OpenDocument Master Document
Text
.odo
Online Operating System Write Document
Text
.odp
OpenDocument Presentation
Data
.ods
OpenDocument Spreadsheet
Spreadsheet
.odt
OpenDocument Text Document
Text
.odx
BizTalk Server Orchestration File
Data
.oeaccount
Windows Mail Account File
Data
.oeb
Outlook Express Backup Wizard File
Backup
.oem
OEM Setup File
Settings
.oex
Opera Extension
Plugin
.ofc
Open Financial Connectivity File
Data
.off
Object File Format
3D Image
.ofl
Ots File List
Text
.ofm
OmniForm Form
Data
.oft
Outlook File Template
Data
.ofx
Open Financial Exchange File
Data
.oga
Ogg Vorbis Audio File
Audio
.ogf
S.T.A.L.K.E.R. Model File
3D Image
.ogg
Ogg Vorbis Compressed Audio File
Audio
.ogm
Ogg Media File
Video
.ognc
Dynamic Web Page
Web
.ogv
Ogg Vorbis Video File
Video
.ogx
Ogg Vorbis Multiplexed Media File
Video
.ogz
Cube 2 Map File
Game
.okt
Oktalyzer Module
Audio
.ola
Online Access File
Layout
.olb
OLE Object Library
Misc
.old
Backup File
Backup
.olk
Outlook Address Book File
Data
.olk14category
Outlook Category File
Data
.olk14contact
Outlook Contact File
Data
.olk14event
Outlook Calendar Event File
Data
.olk14group
Outlook Group File
Data
.olk14mailaccount
Outlook Mail Account File
Data
.olk14message
Outlook Message File
Data
.olk14msgsource
Outlook Email Message Data File
Data
.olk14note
Outlook Note File
Data
.olk14pref
Outlook Preferences File
Settings
.olk14signature
Outlook Signature File
Data
.olk14task
Outlook Task File
Data
.oma
Sony OpenMG Music File
Audio
.omcs
Office Manager Access File
Data
.omf
Open Media Framework File
Audio
.omg
OpenMG Audio File
Audio
.omo
OMake Object File
Developer
.omp
Office Manager Document Archive
Data
.omx
OtsAV Media Library Information File
Audio
.ond
Lotus Notes Encapsulated Memo File
Data
.one
OneNote Document
Data
.onepkg
Microsoft OneNote Package File
Backup
.onetoc2
Microsoft OneNote Table of Contents File
Settings
.oo3
OmniOutliner 3 File
Data
.opd
OmniPage Document
Layout
.openbsd
OpenBSD Readme File
Text
.opf
Open Packaging Format File
Misc
.opml
Outline Processor Markup Language File
Web
.ops
Office Profile Settings File
Settings
.opt
Options File
Settings
.opts
Linux Configuration Options File
Settings
.opx
OrgPlus Org Chart
Data
.or2
Lotus Organizer 2 File
Data
.or3
IBM Lotus Organizer 97 File
Data
.or4
IBM Lotus Organizer 97 GS File
Data
.or5
Lotus Organizer 5 File
Data
.or6
Lotus Organizer 6 File
Data
.orderedtest
Visual Studio Ordered Test File
Developer
.orf
Olympus RAW File
Raster Image
.org
Lotus Organizer File
Data
.ori
Original File
Backup
.ort
Rich Text Editor Document
Text
.orx
RadiantOne VDS Database Schema
Database
.osax
AppleScript Scripting Addition
Plugin
.osp
OpenShot Project File
Video
.oss
Microsoft Outlook Saved Search File
Settings
.ost
Outlook Offline Folder File
Misc
.osx
PowerPC Executable File
Executable
.osz
Osu! Beatmap File
Game
.ota
OTA Bitmap Image
Raster Image
.otb
Nokia Over The Air Bitmap Image
Raster Image
.otc
OpenDocument Chart Template
Misc
.otf
OpenType Font
Font
.otg
OpenDocument Graphic Template
Vector Image
.oth
OpenDocument HTML Template
Web
.oti
OpenDocument Image Template
Raster Image
.otl
NoteTab Outline File
Data
.otln
Opal Outline File
Data
.otm
Outlook Macro File
Executable
.otp
OpenDocument Presentation Template
Data
.ots
OpenDocument Spreadsheet Template
Spreadsheet
.ott
OpenDocument Text Document Template
Text
.out
Output File
Data
.ov2
TomTom Points of Interest Database
GIS
.ova
Octava Musical Score
Data
.ove
Overture Musical Score
Audio
.ovf
Open Virtualization File
Data
.ovl
Rollercoaster Tycoon 3 Sprites File
Game
.ovpn
OpenVPN Configuration File
Settings
.ovw
Logic Pro Overview File
Audio
.owc
OutWit Catch Database
Database
.owg
OutWit Gear Database
Misc
.owl
OWL Source Code File
Developer
.owm
OutWit XML Mashup File
Misc
.oxt
OpenOffice.org Extension
Plugin
.oyx
Approach Alternate Database Index
Backup
.oz
Opera Job Management Compressed File
Compressed