File extension	The full name of the file	Popularity
File extension O	Compiled Object File
File extension OAB	Offline Address Book
File extension OAM	Adobe Edge Animate Widget File
File extension OAR	OpenSimulator Archive
File extension OB!	Orbit Downloader Incomplete Download
File extension OBB	Android Opaque Binary Blob File
File extension OBD	Office Binder Document
File extension OBI	Outlook RSS Subscription File
File extension OBJ	Wavefront 3D Object File
File extension OBML	Opera Mini Saved Web Page
File extension OBP	Bryce Object File
File extension OBT	Openbox Theme File
File extension OBW	Superior Drummer Sounds File
File extension OBZ	Compressed 3D Object File
File extension OC3	openCanvas 3 Event File
File extension OC4	openCanvas 4 Event File
File extension OC5	openCanvas 5 Event File
File extension OCA	Custom Control Library Type File
File extension OCC	Oceanic DataPool File
File extension OCD	OCAD Map File
File extension OCE	Open Catalog Extension Connection File
File extension OCI	openCanvas Image
File extension OCR	FAXGrapper Fax Text File
File extension OCT	Radiance Octree File
File extension OCTEST	Xcode Objective-C Unit Test Bundle
File extension OCX	ActiveX Control
File extension OD	Mascopt Graph Request File
File extension ODB	OpenDocument Database
File extension ODC	Office Data Connection File
File extension ODF	OpenDocument Formula
File extension ODG	OpenDocument Graphic File
File extension ODI	OpenDocument Image
File extension ODIF	Open Document Interchange Format
File extension ODL	Object Description Language File
File extension ODM	OverDrive Media File
File extension ODO	Online Operating System Write Document
File extension ODP	OpenDocument Presentation
File extension ODS	OpenDocument Spreadsheet
File extension ODT	OpenDocument Text Document
File extension ODX	BizTalk Server Orchestration File
File extension OEACCOUNT	Windows Mail Account File
File extension OEB	Outlook Express Backup Wizard File
File extension OEM	OEM Setup File
File extension OEX	Opera Extension
File extension OFC	Open Financial Connectivity File
File extension OFF	Object File Format
File extension OFL	Ots File List
File extension OFM	Melco Embroidery File
File extension OFR	OptimFROG Audio File
File extension OFT	Outlook File Template
File extension OFX	Open Financial Exchange File
File extension OGA	Ogg Vorbis Audio File
File extension OGF	S.T.A.L.K.E.R. Model File
File extension OGG	Ogg Vorbis Audio File
File extension OGM	Ogg Media File
File extension OGNC	Dynamic Web Page
File extension OGS	Origons Movie Data File
File extension OGV	Ogg Video File
File extension OGX	Ogg Vorbis Multiplexed Media File
File extension OGZ	Cube 2 Map File
File extension OKT	Oktalyzer Module
File extension OLA	Online Access File
File extension OLB	OLE Object Library
File extension OLD	Backup File
File extension OLK	Outlook Address Book File
File extension OLK14CATEGORY	Outlook Category File
File extension OLK14CONTACT	Outlook Contact File
File extension OLK14EVENT	Outlook Calendar Event File
File extension OLK14GROUP	Outlook Group File
File extension OLK14MAILACCOUNT	Outlook Mail Account File
File extension OLK14MESSAGE	Outlook Message File
File extension OLK14MSGSOURCE	Outlook Email Message Data File
File extension OLK14NOTE	Outlook Note File
File extension OLK14PREF	Outlook Preferences File
File extension OLK14SIGNATURE	Outlook Signature File
File extension OLK14TASK	Outlook Task File
File extension OLM	Mac Outlook Database File
File extension OLT	Orbit Downloads List
File extension OMA	Sony OpenMG Music File
File extension OMCS	Office Manager Access File
File extension OMF	Open Media Framework File
File extension OMG	O&O DiskImage backup file
File extension OMO	OMake Object File
File extension OMOD	Oblivion Mod Archive
File extension OMP	Office Manager Document Archive
File extension OMX	OtsAV Media Library Information File
File extension OND	Lotus Notes Encapsulated Memo File
File extension ONE	OneNote Document
File extension ONEPKG	Microsoft OneNote Package File
File extension ONETOC2	Microsoft OneNote Table of Contents File
File extension ONT	theWord Bible Text Module
File extension ONTX	theWord Encrypted Bible Text Module
File extension OO3	OmniOutliner 3 File
File extension OOK	Arachne Batch Script
File extension OPD	Durango Interferometry Software document file
File extension OPDOWNLOAD	Opera Partially Downloaded File
File extension OPENBSD	OpenBSD Readme File
File extension OPF	Open Packaging Format File
File extension OPJ	OrCAD Project File
File extension OPLC	Nokia Operator Logo File
File extension OPML	Outline Processor Markup Language File
File extension OPS	Office Profile Settings File
File extension OPT	MySQL database data file
File extension OPTIONS	SE-SOFT Configuration File
File extension OPTS	Linux Configuration Options File
File extension OPUS	Opus Audio File
File extension OPX	OrgPlus Org Chart
File extension OQY	Excel OLAP Query File
File extension OR2	Lotus Organizer 2 File
File extension OR3	IBM Lotus Organizer 97 File
File extension OR4	IBM Lotus Organizer 97 GS File
File extension OR5	Lotus Organizer 5 File
File extension OR6	Lotus Organizer 6 File
File extension ORA	Oracle Database Configuration File
File extension ORC	CSound orchestra midi file
File extension ORDEREDTEST	Visual Studio Ordered Test File
File extension ORE	Ore Executable File
File extension ORF	Olympus RAW File
File extension ORG	Lotus Organizer File
File extension ORI	Original File
File extension ORIG	Original File
File extension ORT	COREX orthographic transcriptions file
File extension ORV	Oracom Video File
File extension ORX	RadiantOne VDS Database Schema
File extension OS	ObjectScript Source File
File extension OSAX	FindFile OSAX mac file type
File extension OSB	OpenStreetMap Binary Map File
File extension OSC	Remote Installation Services client installation wizard file
File extension OSDX	Search Connector Description File
File extension OSK	Osu! Skin File
File extension OSM	OpenStreetMap Map File
File extension OSP	OpenShot Project File
File extension OSR	Osu! Replay File
File extension OSS	Microsoft Outlook Saved Search File
File extension OST	Outlook Offline Folder File
File extension OSU	Osu! Script
File extension OSX	PowerPC Executable File
File extension OSZ	Osu! Beatmap File
File extension OT	theWord Old Testament Text Module
File extension OTA	OTA Bitmap Image
File extension OTB	Nokia Over The Air Bitmap Image
File extension OTC	OpenDocument Chart Template
File extension OTF	OpenType Font
File extension OTG	OpenDocument Graphic Template
File extension OTH	OpenDocument HTML Template
File extension OTI	OpenDocument Image Template
File extension OTL	NoteTab Outline File
File extension OTLN	Opal Outline File
File extension OTM	Outlook Macro File
File extension OTP	OpenDocument Presentation Template
File extension OTRKEY	OnlineTvRecorder Encoded Video File
File extension OTS	OpenDocument Spreadsheet Template
File extension OTT	OpenDocument Document Template
File extension OTX	theWord Encrypted Old Testament Text Module
File extension OTZ	OpenLP Theme File
File extension OUT	Output File
File extension OV2	TomTom Points of Interest Database
File extension OVA	Open Virtual Appliance
File extension OVE	Overture Musical Score
File extension OVF	Open Virtualization File
File extension OVL	Rollercoaster Tycoon 3 Data File
File extension OVP	The Overlay Maker Package File
File extension OVPN	OpenVPN Configuration File
File extension OVR	The Overlay Maker Overlay File
File extension OVW	Logic Pro Overview File
File extension OW	Free Pascal 1.0.x Object File
File extension OWC	OutWit Catch Database
File extension OWG	OutWit Gear Database
File extension OWL	OWL Source Code File
File extension OWM	OutWit XML Mashup File
File extension OXPS	OpenXPS File
File extension OXT	Apache OpenOffice Extension
File extension OXYGENE	Oxygene Project File
File extension OYX	Approach Alternate Database Index
File extension OZ	Opera Job Management Compressed File