| os3 | OS/2 Warp 3 System Data |
| osa | JD Edwards EnterpriseOne OSA File |
| osbx | OneSafe Backup Archive |
| osc | OpenStreetMap Change |
| oscd | TouchOSC Interface Editor Template |
| oset | Marmoset OSET Data |
| osf | Opera Skin Data |
| osh | IBM InfoSphere Schema Data |
| osi | Osiris Data |
| osis | Open Scriptural Information Standard Data |
| osl | OckamSoft 4 Log |
| osl | Orbit Symbol Library |
| osm | OpenStreetMap Map |
| osm | WaterGEMS Outage Segmentation Results Data |
| ospx | Dell KACE Patch Definition FIle |
| oss | Microsoft Office Outlook Saved Search |
| ost | Microsoft Outlook Offline Folder File |
| osw | ONESWING Contents Data |
| osw | Powernet Waveform File |
| osz | OpenLP Service File |
| ot | Atari Jaguar TOM Object |
| ot | TheWord Old Testament Text Module |
| otdr | Yokogawa OTDR Data |
| ote | Orbscan Treatment Export Data |
| oth | Microsoft PowerToys Explorer Tool Data |
| oth | Origin Theme Data |
| otl | NoteTab Outline |
| otl | Ventura Publisher Type Foundry Outline Editor Data |
| otlb | Microsoft OLE Type Library |
| otn | OmniPage Training |
