| 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 |
| otodot | OTOsuite Custom Report Data |
| otoset | OTOsuite Settings Data |
| otp | Top Secret Crypto Gold One Time Pad Key |
| otr | IRIX Compiled File-type Rules Data |
| otv | Rotor-Disc OTV Kit OTV Calibration Data |
| otv5key | onlineTV Registration Key Data |
| otvdatafile | onlineTV Data |
| otvlangfile | onlineTV Language Data |
| otx | theWord Encrypted Old Testament Text Module |
| otz | OpenLP Theme File |
| oub | Oubliette Data |
