| Extension | File Description |
| .ONI | Uncommon Files |
| .ONJ | Uncommon Files |
| .ONK | Uncommon Files |
| .ONL | Uncommon Files |
| .ONLINE | Uncommon Files |
| .ONM | Uncommon Files |
| .ONN | Uncommon Files |
| .ONO | Uncommon Files |
| .ONP | Uncommon Files |
| .ONQ | Uncommon Files |
| .ONR | Uncommon Files |
| .ONS | Uncommon Files |
| .ONT | Uncommon Files |
| .ONU | Uncommon Files |
| .ONV | Uncommon Files |
| .ONW | Uncommon Files |
| .ONX | Uncommon Files |
| .ONY | Uncommon Files |
| .ONZ | Uncommon Files |
| .OO | Uncommon Files |
| .OO0 | Uncommon Files |
| .OO1 | Uncommon Files |
| .OO2 | Uncommon Files |
| .OO3 | Uncommon Files |
| .OO4 | Uncommon Files |
| .OO5 | Uncommon Files |
| .OO6 | Uncommon Files |
| .OO7 | Uncommon Files |
| .OO8 | Uncommon Files |
| .OO9 | Uncommon Files |
| .OOA | Uncommon Files |
| .OOB | Uncommon Files |
| .OOC | Uncommon Files |
| .OOD | Uncommon Files |
| .OOE | Uncommon Files |
| .OOF | Uncommon Files |
| .OOG | Uncommon Files |
| .OOH | Uncommon Files |
| .OOI | Uncommon Files |
| .OOJ | Uncommon Files |
| .OOK | Uncommon Files |
| .OOL | Uncommon Files |
| .OOM | Uncommon Files |
| .OON | Uncommon Files |
| .OOO | Uncommon Files |
| .OOP | Uncommon Files |
| .OOQ | Uncommon Files |
| .OOR | Uncommon Files |
| .OOS | Uncommon Files |
| .OOT | Uncommon Files |
| .OOU | Uncommon Files |
| .OOV | Uncommon Files |
| .OOW | Uncommon Files |
| .OOX | Uncommon Files |
| .OOY | Uncommon Files |
| .OOZ | Uncommon Files |
| .OP | Uncommon Files |
| .OP0 | Uncommon Files |
| .OP1 | Uncommon Files |
| .OP2 | Uncommon Files |
| .OP3 | Uncommon Files |
| .OP4 | Uncommon Files |
| .OP5 | Uncommon Files |
| .OP6 | Uncommon Files |
| .OP7 | Uncommon Files |
| .OP73 | Uncommon Files |
| .OP8 | Uncommon Files |
| .OP9 | Uncommon Files |
| .OPA | Uncommon Files |
| .OPB | Uncommon Files |
| .OPC | Uncommon Files |
| .OPD | Page Layout Files |
| .OPE | Uncommon Files |
| .OPEN | Uncommon Files |
| .OPENYOURMI | Uncommon Files |
| Extension | File Description |
| .OPF | Data Files |
| .OPG | Uncommon Files |
| .OPH | Uncommon Files |
| .OPI | Uncommon Files |
| .OPJ | CAD Files |
| .OPK | Uncommon Files |
| .OPL | Uncommon Files |
| .OPM | Uncommon Files |
| .OPML | Web Files |
| .OPN | Uncommon Files |
| .OPO | Uncommon Files |
| .OPOS | Uncommon Files |
| .OPP | Uncommon Files |
| .OPQ | Uncommon Files |
| .OPR | Uncommon Files |
| .OPRC | Data Files |
| .OPS | Settings Files |
| .OPT | Settings Files |
| .OPTIONAL | Uncommon Files |
| .OPTNS | Uncommon Files |
| .OPU | Uncommon Files |
| .OPUS | Uncommon Files |
| .OPV | Uncommon Files |
| .OPW | Uncommon Files |
| .OPX | Data Files |
| .OPY | Uncommon Files |
| .OPZ | Uncommon Files |
| .OQ0 | Uncommon Files |
| .OQ1 | Uncommon Files |
| .OQ2 | Uncommon Files |
| .OQ3 | Uncommon Files |
| .OQ4 | Uncommon Files |
| .OQ5 | Uncommon Files |
| .OQ6 | Uncommon Files |
| .OQ7 | Uncommon Files |
| .OQ8 | Uncommon Files |
| .OQ9 | Uncommon Files |
| .OQA | Uncommon Files |
| .OQB | Uncommon Files |
| .OQC | Uncommon Files |
| .OQD | Uncommon Files |
| .OQE | Uncommon Files |
| .OQF | Uncommon Files |
| .OQG | Uncommon Files |
| .OQH | Uncommon Files |
| .OQI | Uncommon Files |
| .OQJ | Uncommon Files |
| .OQK | Uncommon Files |
| .OQL | Uncommon Files |
| .OQM | Uncommon Files |
| .OQN | Uncommon Files |
| .OQO | Uncommon Files |
| .OQP | Uncommon Files |
| .OQQ | Uncommon Files |
| .OQR | Uncommon Files |
| .OQS | Uncommon Files |
| .OQT | Uncommon Files |
| .OQU | Uncommon Files |
| .OQV | Uncommon Files |
| .OQW | Uncommon Files |
| .OQX | Uncommon Files |
| .OQY | Database Files |
| .OQZ | Uncommon Files |
| .OR | Uncommon Files |
| .OR0 | Uncommon Files |
| .OR1 | Uncommon Files |
| .OR2 | Data Files |
| .OR3 | Data Files |
| .OR4 | Data Files |
| .OR5 | Data Files |
| .OR6 | Data Files |
| .OR7 | Uncommon Files |
| .OR8 | Uncommon Files |
| .OR9 | Uncommon Files |
| .ORA | Database Files |
