| Extension | File Type | Files scanned | Contributor |
| O | ELF Executable and Linkable format (generic) | 58 | |
| O3M | Ortho 3D Model | 2 | |
| OAD | Notaro document | 13 | |
| OB3 | ORTIM Zeit data | 289 | Scrapie |
| OBD | Office Binder Document | 9 | |
| OBJ | Blender 3D object | 9 | |
| OBJ | CADRazor 3d model | 32 | |
| OBJ | Microsoft Basic 7.1 compiled object code | 88 | |
| OBJ | Microsoft VBDOS 1.0 compiled object code | 57 | |
| OBJ | OMF - Relocatable Object Module Format | 9 | |
| OBJ | UVMapper object | 6 | |
| OBJ/A | Common Object File Format (COFF) Library | 133 | |
| OBP | Bryce Object Library | 28 | |
| OBPACK | ObjectBar theme | 4 | |
| OCC | DB/TextWorks Database Terms and Words | 7 | |
| OCI | OpenCanvas Image | 6 | |
| OCT | Radiance Octree | 4 | |
| OCX | Windows ActiveX control | 589 | |
| ODB | OpenDocument DataBase | 6 | |
| ODG | OpenDocument Graphics document | 21 | |
| ODM | OpenDocument Master Text document | 8 | |
| ODM | OverDrive Media console media control | 9 | |
| ODP | OpenDocument Presentation | 22 | |
| ODS | OpenDocument Spreadsheet document | 27 | |
| ODT | MindRender VREK Object File Format | 5 | |
| ODT | OpenDocument Text document | 83 | |
| ODV | Ocean Data View data (TXT) | 19 | |
| OFC | Open Financial Connectivity file | 3 | |
| OFM | OmniForm Form | 32 | |
| OFR | OptimFROG encoded Audio | 56 | |
| OFT | Outlook Form Template | 57 | |
| OFW | TopLevel Forms Form | 19 | |
| OGG | OGG Vorbis Audio | 35 | |
| OGG | OGG stream (generic) | 35 | |
| OGM | OGG Media stream | 4 | |
| OGV | Ogg Vorbis Video | 9 | |
| OKT | Oktalyzer Module | 36 | |
| OLB | OrCAD PSpice Capture Symbols Models | 76 | |
| OLRW/OLR | Openlab Raw Format | 4 | |
| OLSR | olsrd configuration | 2 | |
| OMA | Sony OpenMG Audio (SonicStage) | 67 | Ildar Ibatullin |
| OMF | Open Source Metadata Framework | 11 | | NEW! |
| OMF/OMFI | Open Media Framework Interchange | 7 | | NEW! |
| OMOD | OpenMRS Module | 10 | |
| OMX | OMAX Make tool path data | 4 | |
| OMX | OMAX Make tool path data (with rem) | 3 | |
| OMX | Ots Media Library | 4 | |
| OND | Lotus Notes Database | 6 | |
| ONE | Microsoft OneNote note | 6 | |
| ONE | SEMONE compressed archive | 7 | |
| ONEPKG | Microsoft OneNote Package | 8 | |
| OOP | OOP compressed archive | 4 | |
| OP2 | Stanford Research Systems Optimlelt file | 2 | |
| OPC | Office Upgrade Control file | 5 | |
| OPD | Durango Interferometry data | 18 | |
| OPD | OmniPage Document | 18 | |
| OPF | Obsidium Project File | 6 | .Cryorb |
| OPF | OmniPass encrypted | 4 | |
| OPF | Open Packaging Format manifest | 13 | |
| OPI | OmniPass exported user | 2 | |
| OPJ | OrCAD Project | 17 | |
| OPJ | Origin Project | 26 | |
| OPK | Origin Pack file | 5 | |
| OPML | OPML - Outline Processor Markup Language | 20 | |
| OPPC/OPPS3 | Darksiders game data package | 5 | |
| OPS | Office Profile-Settings (v1.1) | 4 | |
| OPS | Office Profile-Settings (v10.0) | 3 | |
| OPS | Office Profile-Settings (v11.0) | 6 | |
| OPT | Neuratron PhotoScore Document | 2 | |
| OPX | OrgPlus Organization Chart | 16 | |
| OPY | OptiY Model | 14 | TBeu |
| ORA | OpenRaster image | 6 | |
| ORC | Csound orchestra instrument | 10 | |
| ORF | Olympus digital camera RAW image | 7 | |
| OSB | OpenStreetMap Binary map data | 2 | |
| OSC | OpenStreetMap Change data | 3 | |
| OSC | OptionScope Worksheet | 2 | |
| OSM | OpenStreetMap XML Data | 10 | |
| OSP | Obi Synth Pack Music | 17 | |
| OST | Outlook Exchange Offline Storage | 20 | Jonathan Marsh |
| OTF | OpenType - CFF compact font format | 8 | |
| OTO | AutoPano project | 2 | |
| OTP | OpenDocument Presentation Template | 8 | |
| OTRKEY | OnlineTVRecorder (OTR) Keyfile | 5 | Icfu |
| OTS | OpenDocument Spreadsheet template | 39 | |
| OTT | OpenDocument Text Document template | 336 | |
| OUT | Lua 4.0 bytecode | 2 | |
| OUT | Lua 5.0 bytecode | 2 | |
| OUT | Lua bytecode (generic) | 10 | |
| OUT/TXT | Wireshark traffic log | 20 | |
| OVE | Cakewalk Overture Score | 12 | |
| OVL | RollerCoaster Tycoon 3 game data | 3 | |
| OVR | Borland Overlay | 6 | |
| OVX | Psion OVAL Control | 12 | |
| OXYGENE | Oxygene Project | 5 | |
| OXYGENE | Oxygene Project (UTF-8) | 10 | |
| OZF | Mozart functor | 132 | |
| OZF2 | OziExplorer Map | 11 | |
| OZFX3 | OziExplorer Map | 3 | |
| OZV | ORTIM Zeit project | 6 | Scrapie |
