| osa | JD Edwards EnterpriseOne OSA File |
| osbx | OneSafe Backup Archive |
| osc | OpenStreetMap Change |
| oscd | TouchOSC Interface Editor Template |
| oset | Marmoset OSET Data |
| osh | IBM InfoSphere Schema Data |
| osi | Osiris Data |
| osis | Open Scriptural Information Standard Data |
| osm | OpenStreetMap Map |
| ospx | Dell KACE Patch Definition FIle |
| oss | Microsoft Office Outlook Saved Search |
| ost | Microsoft Outlook Offline Folder File |
| osw | Powernet Waveform File |
| osz | OpenLP Service File |
| ot | TheWord Old Testament Text Module |
| otdr | Yokogawa OTDR Data |
| ote | Orbscan Treatment Export Data |
| oth | Origin Theme Data |
| otl | NoteTab Outline |
| otn | OmniPage Training |
| otv | Rotor-Disc OTV Kit OTV Calibration Data |
| 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 |
