| otx | theWord Encrypted Old Testament Text Module |
| otz | OpenLP Theme File |
| oub | Oubliette Data |
| oud | Adlib Express User Defined Dictionary |
| ouf | Otravi Unit Frames Data |
| out | Altair FEKO POSTFEKO ASCII Output Data |
| out | FAST Output Data |
| outb | FAST Binary Output Data |
| outjob | Altium Designer OutputJob Data |
| outline | FontTwister Letter Outline Data |
| ouv | Sphinx Open Ended Responses Data |
| oux | PCCharge Transaction Response XML File |
| ov | OpenInsight Overflow Data |
| ov1 | MS-DOS Overlay Data |
| ov2 | MS-DOS Overlay Data |
| ov2 | TomTom POI Database |
| ov3 | MS-DOS Overlay Data |
| ov4 | MS-DOS Overlay Data |
| ovb | OnkoS Vocabulary Backup Data |
| ovd | OverDisk Data |
| overlay | Streamlabs OBS Overlay Data |
| ovf | Open Virtualization Format Data |
| ovi | PC-9801 FM System Data |
| ovisuite | Nokia Ovi Suite Data |
| ovl | E-Sword v8 Overlay Data |
| ovlx | E-Sword v9 Overlay Data |
| ovm | BHV Tonstudio Data |
| ovml | Ovidentia Markup Language Document |
| ovolog | ooVoo Log File |
| ovs | ARTA Overlay Data |
| ovt | Orbit Vector File Tiled |
| ovw | Cubase WAVE File Overview |
| owc | OutWit Hub OutWit Catch Database |
| owc | WAsP Observed Wind Climate |
| owf | OSRAM Waveform File |