| Abbreviation | Definition |
|---|---|
| OPA | Psion OPL Object file |
| OPL | OPL language source code file (Psion) |
| OPN | Active options (Exact) |
| OPO | Psion OPL Object file |
| OPT | Optimize support file (QEMM) |
| OPT | Optional |
| OPT | Options |
| OPW | Organization chart (Org Plus for Windows) |
| OPX | Chart (MS Organization) |
| OPX | Inactive options (Exact) |
| OR8 | Oracle 8 database model report (Rational Rose Oracle8) |
| ORA | Parameter file (Oracle) |
| ORG | Lotus Organizer Calendar file |
| ORG | Microcal Origin plot file |
| OS2 | OS/2 Important File |
| OSF | Open Source File |
| OSF | Outrage Sound Format |
| OSS | MicroSoft Office Search result |
| OST | Offline Storage Tables |
| OTF | Open Ttype Font File |