| Extension | Summary |
|---|
| .O | Compiled Object File |
| .OAB | Offline Address Book |
| .OAM | Adobe Edge Animate Widget File |
| .OAP | OutSystems Application Package |
| .OAR | OpenSimulator Archive |
| .OAT | Android Optimized Application File |
| .OB! | Orbit Downloader Incomplete Download |
| .OB2 | Oberon-2 Source Code File |
| .OBB | Android Opaque Binary Blob File |
| .OBD | Office Binder Document |
| .OBF | OsmAnd Offline Map File |
| .OBI | Outlook RSS Subscription File |
| .OBJ | Wavefront 3D Object File |
| .OBK | OrangeCD Backup File |
| .OBL | openCanvas Brush File |
| .OBML | Opera Mini Saved Webpage |
| .OBML15 | Opera Mini Saved Webpage |
| .OBML16 | Opera Mini Saved Webpage |
| .OBP | Bryce Object File |
| .OBT | Openbox Theme File |
| .OBW | Superior Drummer Sounds File |
| .OBZ | Compressed 3D Object File |
| .OC3 | openCanvas 3 Event File |
| .OC4 | openCanvas 4 Event File |
| .OC5 | openCanvas 5 Event File |
| .OCA | Custom Control Library Type File |
| .OCC | Oceanic DataPool File |
| .OCD | OCAD Map File |
| .OCE | Open Catalog Extension Connection File |
| .OCI | openCanvas Image |
| .OCIMF | OCIMF HVPQ File |
| .OCR | Subtitle Workshop OCR Script |
| .OCT | Radiance Octree File |
| .OCTEST | Xcode Objective-C Unit Test Bundle |
| .OCX | ActiveX Control |
| .OD | Mascopt Graph Request File |
| .ODB | ABAQUS Database |
| .ODC | Office Data Connection File |
| .ODEX | Optimized Dalvik Executable |
| .ODF | OpenDocument Formula |
| .ODG | OpenDocument Graphic File |
| .ODI | OpenDocument Image |
| .ODIF | Open Document Interchange Format |
| .ODIN | Locky Ransomware Encrypted File |
| .ODL | Object summary Language File |
| .ODM | OverDrive Media File |
| .ODO | Online Operating System Write Document |
| .ODOC | Synology Office Document Pointer |
| .ODP | OpenDocument Presentation |
| .ODS | Outlook Express 5 Mailbox |
| .ODT | Origin Dialog Theme File |
| .ODTTF | Obfuscated OpenType Font |
| .ODX | BizTalk Server Orchestration File |
| .OEACCOUNT | Windows Mail Account File |
| .OEB | Outlook Express Backup Wizard File |
| .OEBZIP | Zipped Open eBook File |
| .OEM | OEM Setup File |
| .OESAVESTATE | OpenEmu Saved Game |
| .OEX | Opera Extension |
| .OFC | Open Financial Connectivity File |
| .OFD | OfficeForms Document |
| .OFF | Object File Format |
| .OFFICEUI | Microsoft Office UI Customization File |
| .OFL | Ots File List |
| .OFM | Melco Embroidery File |
| .OFP | Origin Function Plot Theme File |
| .OFR | SMS Offer Manager File |
| .OFT | Outlook File Template |
| .OFX | Open Financial Exchange File |
| .OGA | Ogg Vorbis Audio File |
| .OGF | S.T.A.L.K.E.R. Model File |
| .OGG | Ogg Vorbis Audio File |
| .OGGSTR | Godot Engine Ogg Audio File |
| .OGGU | Origin Unicode Graph File |
| .OGM | Ogg Media File |
| .OGMU | Origin Unicode Matrix File |
| .OGNC | Dynamic Web Page |
| .OGS | Origons Movie Data File |
| .OGV | Ogg Video File |
| .OGW | Origin Workbook File |
| .OGWU | Origin Unicode Workbook File |
| .OGX | Ogg Vorbis Multiplexed Media File |
| .OGZ | Cube 2 Map File |
| .OIF | Origin Import Filter File |
| .OIS | Origin Analyses Theme File |
| .OIV | OpenIV Mod Package File |
| .OKT | Oktalyzer Module |
| .OL | Valve Hammer Object Layout File |
| .OLA | Online Access File |
| .OLB | OLE Object Library |
| .OLD | Backup File |
| .OLK | Outlook Address Book File |
| .OLK14CATEGORY | Outlook Category File |
| .OLK14CONTACT | Outlook Contact File |
| .OLK14EVENT | Outlook Calendar Event File |
| .OLK14GROUP | Outlook Group File |
| .OLK14MAILACCOUNT | Outlook Mail Account File |
| .OLK14MESSAGE | Outlook Message File |
| .OLK14MSGATTACH | Outlook 2011 Email Message Attachment File |
| .OLK14MSGSOURCE | Outlook Email Message Data File |
| .OLK14NOTE | Outlook Note File |
| .OLK14PREF | Outlook Preferences File |
| .OLK14SIGNATURE | Outlook Signature File |
| .OLK14TASK | Outlook Task File |
| .OLK15MSGATTACH | Outlook 2016 Email Message Attachment File |
| .OLL | TrialDirector Object Load List |
| .OLM | Mac Outlook Database File |
| .OLP | Office Live Package |
| .OLT | Orbit Downloads List |
| .OMA | Sony OpenMG Music File |
| .OMCS | Office Manager Access File |
| .OMF | OMF Interchange Image File |
| .OMFL | Open Multiple Files File List |
| .OMG | Image File |
| .OML | OutSystems Modeling Language File |
| .OMO | OMake Object File |
| .OMOD | Oblivion Mod Archive |
| .OMP | OpenMind Windows Document |
| .OMS | HP Printer Substrate Presets Package File |
| .OMX | OtsAV Media Library Information File |
| .OND | Lotus Notes Encapsulated Memo File |
| .ONE | OneNote Document |
| .ONEPKG | Microsoft OneNote Package File |
| .ONETOC | Microsoft OneNote Table of Contents File |
| .ONETOC2 | Microsoft OneNote Table of Contents File |
| .ONT | theWord Bible Text Module |
| .ONTX | theWord Encrypted Bible Text Module |
| .OO3 | OmniOutliner 3 File |
| .OO3TEMPLATE | OmniOutliner Template |
| .OOK | Arachne Batch Script |
| .OP | OpenPAT Report |
| .OP2 | Nastran Output File |
| .OP4 | Nastran Output File |
| .OPAL | Microsoft Office User Settings File |
| .OPAX | Microsoft Office User Settings File |
| .OPD | OmniPage Document |
| .OPDOWNLOAD | Opera Partially Downloaded File |
| .OPEICO | Opeico Text File |
| .OPENBSD | OpenBSD Readme File |
| .OPF | FlipAlbum File |
| .OPJ | OrCAD Project File |
| .OPJU | Origin Unicode Project |
| .OPK | GCW Zero Open Package |
| .OPLC | Nokia Operator Logo File |
| .OPML | Outline Processor Markup Language File |
| .OPS | Office Profile Settings File |
| .OPT | Options File |
| .OPTIONS | SE-SOFT Configuration File |
| .OPTS | Linux Configuration Options File |
| .OPUS | Opus Audio File |
| .OPX | FlipAlbum Photo Album File |
| .OQY | Microsoft Excel OLAP Query |
| .OR2 | Lotus Organizer 2 File |
| .OR3 | IBM Lotus Organizer 97 File |
| .OR4 | IBM Lotus Organizer 97 GS File |
| .OR5 | Lotus Organizer 5 File |
| .OR6 | Lotus Organizer 6 File |
| .ORA | Oracle Database Configuration File |
| .ORB | Original eBook Reader File |
| .ORC | Voyetra Digital Orchestrator File |
| .ORDEREDTEST | Visual Studio Ordered Test File |
| .ORE | Ore Executable File |
| .ORF | Olympus RAW File |
| .ORG | Organya Music File |
| .ORI | Original File |
| .ORIG | Original File |
| .ORT | Rich Text Editor Document |
| .ORV | Oracom Video File |
| .ORX | RadiantOne VDS Database Schema |
| .OS | ObjectScript Source File |
| .OSAX | AppleScript Scripting Addition |
| .OSB | OpenStreetMap Binary Map File |
| .OSC | Obix Source Code File |
| .OSDX | Search Connector summary File |
| .OSK | osu! Skin File |
| .OSM | OpenStreetMap Map File |
| .OSP | OpenShot Project File |
| .OSR | osu! Replay File |
| .OSS | Microsoft Outlook Saved Search File |
| .OST | Outlook Offline Data File |
| .OSU | Osu! Script |
| .OSX | PowerPC Executable File |
| .OSZ | osu! Beatmap File |
| .OSZ2 | osu! Submitted Beatmap File |
| .OT | theWord Old Testament Text Module |
| .OTA | OTA Bitmap Image |
| .OTB | Nokia Over The Air Bitmap Image |
| .OTC | OpenDocument Chart Template |
| .OTF | OpenDocument Formula Template |
| .OTG | OpenDocument Graphic Template |
| .OTH | Origin Graph Theme File |
| .OTI | OpenDocument Image Template |
| .OTL | NoteTab Outline File |
| .OTLN | Opal Outline File |
| .OTM | Origin Matrix Template |
| .OTMU | Origin Unicode Matrix Template |
| .OTP | Origin Graph Template |
| .OTPU | Origin Unicode Graph Template |
| .OTRKEY | OnlineTvRecorder Encoded Video File |
| .OTS | OpenDocument Spreadsheet Template |
| .OTT | OpenDocument Document Template |
| .OTW | Origin Workbook Template |
| .OTWU | Origin Unicode Workbook Template |
| .OTX | theWord Encrypted Old Testament Text Module |
| .OTZ | OpenLP Theme File |
| .OUT | Compiled Executable File |
| .OV2 | TomTom Points of Interest Database |
| .OVA | Octava Musical Score |
| .OVE | Overture Musical Score |
| .OVF | Open Virtualization File |
| .OVH | OMSI Vehicle Configuration File |
| .OVL | Rollercoaster Tycoon 3 Data File |
| .OVOLOG | ooVoo Log File |
| .OVP | The Overlay Maker Package File |
| .OVPN | OpenVPN Configuration File |
| .OVR | The Overlay Maker Overlay File |
| .OVW | Cubase WAVE Overview File |
| .OVX | Object-based Visual Application Language File |
| .OW | Free Pascal 1.0.x Object File |
| .OWC | OutWit Catch Database |
| .OWG | OutWit Gear Database |
| .OWL | OWL Source Code File |
| .OWM | OutWit XML Mashup File |
| .OWX | OutWit Hub Automator File |
| .OXPS | OpenXPS File |
| .OXT | Apache OpenOffice Extension |
| .OXYGENE | Oxygene Project File |
| .OYX | Approach Alternate Database Index |
| .OZ | Opera Job Management Compressed File |
| .OZB | MU Online Image File |
| .OZIP | OPPO Smartphone Firmware Update |
| .OZJ | MU Online Image File |
| .OZOCODE | Ozobot Program |
| .OZT | MU Online Image File |