| .O | Compiled Object File | 179 |
| .OAB | Offline Address Book | 250 |
| .OAM | Adobe Edge Animate Widget File | 214 |
| .OAP | OutSystems Application Package | 400 |
| .OAR | OpenSimulator Archive | 250 |
| .OAT | Android Optimized Application File | 300 |
| .OB! | Orbit Downloader Incomplete Download | 175 |
| .OB2 | Oberon-2 Source Code File | 360 |
| .OBB | Android Opaque Binary Blob File | 203 |
| .OBD | Office Binder Document | 257 |
| .OBF | OsmAnd Offline Map File | 333 |
| .OBI | Outlook RSS Subscription File | 260 |
| .OBJ | Wavefront 3D Object File | 209 |
| .OBJ | Relocatable Object Code | 221 |
| .OBK | Backup File | 220 |
| .OBML | Opera Mini Saved Web Page | 238 |
| .OBML15 | Opera Mini Saved Web Page | 300 |
| .OBML16 | Opera Mini Saved Web Page | 200 |
| .OBP | Bryce Object File | 189 |
| .OBT | Openbox Theme File | 200 |
| .OBW | Superior Drummer Sounds File | 300 |
| .OBZ | Compressed 3D Object File | 400 |
| .OC3 | openCanvas 3 Event File | 350 |
| .OC4 | openCanvas 4 Event File | 300 |
| .OC5 | openCanvas 5 Event File | 300 |
| .OCA | Custom Control Library Type File | 200 |
| .OCC | Oceanic DataPool File | 400 |
| .OCD | OCAD Map File | 260 |
| .OCE | Open Catalog Extension Connection File | 450 |
| .OCI | openCanvas Image | 280 |
| .OCIMF | OCIMF HVPQ File | 400 |
| .OCR | FAXGrapper Fax Text File | 467 |
| .OCR | Subtitle Workshop OCR Script | 200 |
| .OCT | Radiance Octree File | 350 |
| .OCTEST | Xcode Objective-C Unit Test Bundle | 233 |
| .OCX | ActiveX Control | 198 |
| .OD | Mascopt Graph Request File | 267 |
| .ODB | ABAQUS Database | 286 |
| .ODB | OpenDocument Database | 247 |
| .ODC | Office Data Connection File | 192 |
| .ODC | OpenDocument Chart File | 350 |
| .ODEX | Optimized Dalvik Executable | 258 |
| .ODF | Battlezone Object Definition File | 314 |
| .ODF | OpenDocument Formula | 255 |
| .ODF | Open Document File | 400 |
| .ODG | OpenDocument Graphic File | 266 |
| .ODI | OpenDocument Image | 333 |
| .ODIF | Open Document Interchange Format | 233 |
| .ODIN | Locky Ransomware Encrypted File | 220 |
| .ODL | Object Description Language File | 367 |
| .ODM | OpenDocument Master Document | 227 |
| .ODM | OverDrive Media File | 247 |
| .ODO | Online Operating System Write Document | 320 |
| .ODP | OpenDocument Presentation | 235 |
| .ODS | Outlook Express 5 Mailbox | 267 |
| .ODS | OpenDocument Spreadsheet | 242 |
| .ODT | OOMMF Data Table File | 256 |
| .ODT | OpenDocument Text Document | 245 |
| .ODT | Origin Dialog Theme File | 321 |
| .ODTTF | Obfuscated OpenType Font | 220 |
| .ODX | BizTalk Server Orchestration File | 400 |
| .OEACCOUNT | Windows Mail Account File | 250 |
| .OEB | Open eBook File | 367 |
| .OEB | Outlook Express Backup Wizard File | 350 |
| .OEBZIP | Zipped Open eBook File | 400 |
| .OEM | OEM Setup File | 250 |
| .OEM | OEMEdit Save File | 350 |
| .OEX | Opera Extension | 210 |
| .OFC | Open Financial Connectivity File | 233 |
| .OFF | Object File Format | 180 |
| .OFFICEUI | Microsoft Office UI Customization File | 186 |
| .OFL | Ots File List | 325 |
| .OFM | Melco Embroidery File | 400 |
| .OFM | OmniForm Form | 400 |
| .OFP | Origin Function Plot Theme File | 300 |
| .OFR | OptimFROG Audio File | 380 |
| .OFR | SMS Offer Manager File | 500 |
| .OFT | Outlook File Template | 253 |
| .OFX | Open Financial Exchange File | 245 |
| .OGA | Ogg Vorbis Audio File | 229 |
| .OGF | S.T.A.L.K.E.R. Model File | 333 |
| .OGG | Origin Graph File | 354 |
| .OGG | Ogg Vorbis Audio File | 214 |
| .OGGSTR | Godot Engine Ogg Audio File | 233 |
| .OGGU | Origin Unicode Graph File | 200 |
| .OGM | Origin Matrix File | 217 |
| .OGM | Ogg Media File | 287 |
| .OGMU | Origin Unicode Matrix File | 300 |
| .OGNC | Dynamic Web Page | 300 |
| .OGS | Origin LabTalk Script File | 250 |
| .OGS | Origons Movie Data File | 500 |
| .OGV | Ogg Video File | 253 |
| .OGW | Origin Workbook File | 400 |
| .OGWU | Origin Unicode Workbook File | 300 |
| .OGX | Ogg Vorbis Multiplexed Media File | 271 |
| .OGZ | Cube 2 Map File | 200 |
| .OIF | Origin Import Filter File | 500 |
| .OIS | Origin Analyses Theme File | 400 |
| .OIV | OpenIV Mod Package File | 182 |
| .OKT | Oktalyzer Module | 267 |
| .OL | Valve Hammer Object Layout File | 242 |
| .OLA | Online Access File | 162 |
| .OLB | OrCAD Symbol Library File | 264 |
| .OLB | OLE Object Library | 208 |
| .OLD | Backup File | 233 |
| .OLK | Outlook Address Book File | 179 |
| .OLK14CATEGORY | Outlook Category File | 450 |
| .OLK14CONTACT | Outlook Contact File | 400 |
| .OLK14EVENT | Outlook Calendar Event File | 400 |
| .OLK14GROUP | Outlook Group File | 400 |
| .OLK14MAILACCOUNT | Outlook Mail Account File | 450 |
| .OLK14MESSAGE | Outlook Message File | 260 |
| .OLK14MSGATTACH | Outlook 2011 Email Message Attachment File | 400 |
| .OLK14MSGSOURCE | Outlook Email Message Data File | 233 |
| .OLK14NOTE | Outlook Note File | 400 |
| .OLK14PREF | Outlook Preferences File | 400 |
| .OLK14SIGNATURE | Outlook Signature File | 400 |
| .OLK14TASK | Outlook Task File | 500 |
| .OLK15MSGATTACH | Outlook 2016 Email Message Attachment File | 400 |
| .OLL | TrialDirector Object Load List | 300 |
| .OLM | Mac Outlook Database File | 162 |
| .OLP | Office Live Package | 250 |
| .OLT | Orbit Downloads List | 350 |
| .OMA | Sony OpenMG Music File | 217 |
| .OMCS | Office Manager Access File | 250 |
| .OMF | OMF Interchange Image File | 375 |
| .OMF | Open Media Framework File | 200 |
| .OMG | Image File | 260 |
| .OMG | OpenMG Audio File | 200 |
| .OML | OutSystems Modeling Language File | 400 |
| .OMO | OMake Object File | 225 |
| .OMOD | Oblivion Mod Archive | 300 |
| .OMP | OpenMind Windows Document | 300 |
| .OMP | Office Manager Document Archive | 400 |
| .OMS | HP Printer Substrate Presets Package File | 300 |
| .OMX | OtsAV Media Library Information File | 200 |
| .OND | Lotus Notes Encapsulated Memo File | 400 |
| .ONE | OneNote Document | 248 |
| .ONEPKG | Microsoft OneNote Package File | 220 |
| .ONETOC | Microsoft OneNote Table of Contents File | 500 |
| .ONETOC2 | Microsoft OneNote Table of Contents File | 209 |
| .ONT | theWord Bible Text Module | 250 |
| .ONTX | theWord Encrypted Bible Text Module | 200 |
| .OO3 | OmniOutliner 3 File | 229 |
| .OO3TEMPLATE | OmniOutliner Template | 350 |
| .OOK | Arachne Batch Script | 400 |
| .OP | OpenPAT Report | 267 |
| .OP2 | Nastran Output File | 300 |
| .OP4 | Nastran Output File | 400 |
| .OPAL | Microsoft Office User Settings File | 360 |
| .OPAX | Microsoft Office User Settings File | 312 |
| .OPD | OmniPage Document | 270 |
| .OPD | Durango Document | 450 |
| .OPDOWNLOAD | Opera Partially Downloaded File | 191 |
| .OPEICO | Opeico Text File | 240 |
| .OPENBSD | OpenBSD Readme File | 333 |
| .OPF | Open Packaging Format File | 264 |
| .OPF | FlipAlbum File | 227 |
| .OPF | FlipViewer FlipBook File | 242 |
| .OPJ | OrCAD Project File | 270 |
| .OPJ | Origin Project | 247 |
| .OPJU | Origin Unicode Project | 214 |
| .OPK | GCW Zero Open Package | 260 |
| .OPLC | Nokia Operator Logo File | 300 |
| .OPML | Outline Processor Markup Language File | 227 |
| .OPS | Office Profile Settings File | 250 |
| .OPT | AGT Game Data File | 325 |
| .OPT | Opterecenja File | 156 |
| .OPT | Options File | 223 |
| .OPTIONS | SE-SOFT Configuration File | 267 |
| .OPTS | Linux Configuration Options File | 300 |
| .OPUS | Opus Audio File | 239 |
| .OPX | OrgPlus Org Chart | 214 |
| .OPX | FlipAlbum Photo Album File | 367 |
| .OQY | Excel OLAP Query File | 220 |
| .OR2 | Lotus Organizer 2 File | 500 |
| .OR3 | IBM Lotus Organizer 97 File | 467 |
| .OR4 | IBM Lotus Organizer 97 GS File | 350 |
| .OR5 | Lotus Organizer 5 File | 350 |
| .OR6 | Lotus Organizer 6 File | 300 |
| .ORA | OpenRaster Image File | 314 |
| .ORA | Oracle Database Configuration File | 282 |
| .ORB | Original eBook Reader File | 300 |
| .ORC | Voyetra Digital Orchestrator File | 500 |
| .ORDEREDTEST | Visual Studio Ordered Test File | 300 |
| .ORE | Ore Executable File | 400 |
| .ORF | Olympus RAW File | 264 |
| .ORG | Lotus Organizer File | 272 |
| .ORI | Original File | 323 |
| .ORIG | Original File | 217 |
| .ORT | Rich Text Editor Document | 333 |
| .ORV | Oracom Video File | 300 |
| .ORX | RadiantOne VDS Database Schema | 250 |
| .OS | ObjectScript Source File | 293 |
| .OSAX | AppleScript Scripting Addition | 325 |
| .OSB | OpenStreetMap Binary Map File | 350 |
| .OSB | Osu! Storyboard File | 214 |
| .OSC | OpenStreetMap Change File | 220 |
| .OSC | Obix Source Code File | 450 |
| .OSDX | Search Connector Description File | 233 |
| .OSK | osu! Skin File | 154 |
| .OSM | OpenStreetMap Map File | 221 |
| .OSP | OpenShot Project File | 260 |
| .OSR | osu! Replay File | 186 |
| .OSS | Microsoft Outlook Saved Search File | 200 |
| .OST | Outlook Offline Data File | 248 |
| .OSU | Osu! Script | 244 |
| .OSX | PowerPC Executable File | 264 |
| .OSZ | osu! Beatmap File | 156 |
| .OSZ | OpenLP Service File | 362 |
| .OT | theWord Old Testament Text Module | 350 |
| .OTA | OTA Ringtone File | 400 |
| .OTA | OTA Bitmap Image | 175 |
| .OTB | Nokia Over The Air Bitmap Image | 250 |
| .OTC | OpenDocument Chart Template | 433 |
| .OTF | OpenTuft Tufting Design File | 330 |
| .OTF | OpenType Font | 226 |
| .OTF | OpenDocument Formula Template | 341 |
| .OTG | OpenDocument Graphic Template | 200 |
| .OTH | Origin Graph Theme File | 500 |
| .OTH | OpenDocument HTML Template | 400 |
| .OTI | OpenDocument Image Template | 250 |
| .OTL | NoteTab Outline File | 300 |
| .OTLN | Opal Outline File | 200 |
| .OTM | Outlook Macro File | 167 |
| .OTM | Origin Matrix Template | 240 |
| .OTMU | Origin Unicode Matrix Template | 250 |
| .OTP | Origin Graph Template | 214 |
| .OTP | OpenDocument Presentation Template | 267 |
| .OTPU | Origin Unicode Graph Template | 300 |
| .OTRKEY | OnlineTvRecorder Encoded Video File | 300 |
| .OTS | OtsAV Album File | 314 |
| .OTS | OpenDocument Spreadsheet Template | 262 |
| .OTT | OpenDocument Document Template | 193 |
| .OTW | Origin Workbook Template | 250 |
| .OTWU | Origin Unicode Workbook Template | 400 |
| .OTX | theWord Encrypted Old Testament Text Module | 433 |
| .OTZ | OpenLP Theme File | 250 |
| .OUT | Compiled Executable File | 204 |
| .OUT | Output File | 278 |
| .OV2 | TomTom Points of Interest Database | 386 |
| .OVA | Open Virtual Appliance | 215 |
| .OVA | Octava Musical Score | 300 |
| .OVE | Overture Musical Score | 325 |
| .OVF | Open Virtualization File | 214 |
| .OVH | OMSI Vehicle Configuration File | 300 |
| .OVL | Rollercoaster Tycoon 3 Data File | 250 |
| .OVOLOG | ooVoo Log File | 250 |
| .OVP | The Overlay Maker Package File | 350 |
| .OVPN | OpenVPN Configuration File | 193 |
| .OVR | The Overlay Maker Overlay File | 300 |
| .OVW | Cubase WAVE Overview File | 333 |
| .OVW | Logic Pro Overview File | 200 |
| .OVX | Object-based Visual Application Language File | 500 |
| .OW | Free Pascal 1.0.x Object File | 333 |
| .OWC | OutWit Catch Database | 400 |
| .OWG | OutWit Gear Database | 400 |
| .OWL | OWL Source Code File | 257 |
| .OWM | OutWit XML Mashup File | 233 |
| .OWX | OutWit Hub Automator File | 375 |
| .OXPS | OpenXPS File | 240 |
| .OXT | Apache OpenOffice Extension | 283 |
| .OXYGENE | Oxygene Project File | 250 |
| .OYX | Approach Alternate Database Index | 350 |
| .OZ | Opera Job Management Compressed File | 250 |
| .OZB | MU Online Image File | 300 |
| .OZJ | MU Online Image File | 200 |
| .OZT | MU Online Image File | 200 |