| O | Object file (Unix) |
| O | Object file (Atari) |
| O | Object file (GCC) |
| OAS | Word processor document (Fujitsu OAS)(Japanese) |
| OAZ | Fax (NetFax Manager) |
| OB | Object cut/paste file (IBM LinkWay) |
| OBD | Binder template (Microsoft Office) |
| OBD | Binder (Microsoft Office) |
| OBJ | Object file |
| OBR | Object browser data file (Borland C++) |
| OBS | Script (ObjectScript) |
| OBV | Visual interface (ObjectScript) |
| OBZ | Binder Wizard (Microsoft Office) |
| OCF | Object craft file (Object Craft) |
| OCR | Transcribed fax-to-text file (FAXGrabber) |
| OCX | Object Linking and Embedding (OLE) custom control (Microsoft) |
| ODE | Object data embedding file (Microsoft Office) |
| ODL | Type library source (Visual C++) |
| ODS | Mailbox file (Microsoft Outlook Express) |
| OFD | Form definition (ObjectView) |
| OFF | Object File (3D Mesh) |
| OFN | FileNew file (Microsoft Office) |
| OFT | Template (Microsoft Outlook) |
| OKR | Feldeinteilung module file (3-D Fassade Plus) |
| OKT | Music module (MOD) (Oktalyzer) |
| OLB | Object Library (OLE) (Microsoft) |
| OLB | Object library (VAX) |
| OLD | Backup file (Generic) |
| OLE | OLE object |
| OLI | Text file (Olivetti) |
| OO1 | Voice file (Typhoon) |
| OOGL | Object Oriented Graphics Library |
| OOM | Swap file (Shroom) |
| OPJ | Project file (Orcad Schematic Capture) |
| OPL | Organiser Programming Language source file (Psion/Symbian) |
| OPN | Active options (Exact) |
| OPO | Output executable file (OPL) |
| OPT | Developer Studio file (Microsoft) |
| OPT | Optimize support file (QEMM) |
| OPW | Organization chart (Org Plus for Windows) |
| OPX | Extension DLL (OPL) |
| OPX | Inactive options (Exact) |
| ORA | Parameter file (Oracle) |
| ORC | Script (Oracle 7) |
| ORG | Calendar file (Lotus Organizer) |
| OR2 | Calendar file (Lotus Organizer 2) |
| OR3 | Lotus Organizer 97 file |
| ORA | Configuration file (Oracle 7) |
| OSS | Search file (Microsoft Office) |
| OST | Offline file (Microsoft Exchange/Outlook) |
| OTL | Outline font description (Z-Soft Type Foundry) |
| OTL | Template file (Super NoteTab) (Fookes) |
| OTL | Template file (PrintMaster Gold) |
| OTX | Text file (Olivetti Olitext Plus) |
| OUT | Output file (Microsoft C) |
| OV | Database file (Revelation-DOS) |
| OV1 | Overlay file |
| OV2 | Overlay file |
| OVD | Datafile (ObjectVision) |
| OVG | Converted graphic file (ObjectVision) |
| OVL | Overlay file |
| OVR | Overlay file |
| O$$ | Outfile (Sprint) |
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Le Havre