| orbx | OctaneRender Scene Package |
| ord | OMAX Routed Data File |
| org | Lotus Organizer Data |
| orient | Ribbons Orientation File |
| orj | InterSelect PhotoScan Data |
| orl | Object-Relational Language Data |
| orm | Ormetis Workbench Data |
| oro | 3WayPack T3Rotate Output |
| orp | WinSLAMM ORP Data |
| orq | OCSP Request Data |
| orx | RadiantOne VDS Database Schema |
| ory | Act! Opportunities Query Data |
| 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 |
| 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 |