| 1. | .op | Operation outout (Spice) |
| 2. | .opa | Object file (Psion OPL) |
| 3. | .opj | Project file (Orcad Schematic Capture) |
| 4. | .opl | OPL (Organizer Programming Language) language source code file (Psion) |
| 5. | .opn | Active options (Exact) |
| 6. | .opo | Object file (Psion compiled OPL) |
| 7. | .opt | Developer Studio file (Microsoft) ◊ Optimize support file (QEMM) ◊ Options |
| 8. | .opw | Organization chart (Org Plus for Windows) |
| 9. | .opx | Chart (Microsoft Organization) ◊ Extension DLL (OPL) ◊ Inactive options (Exact) |