| opp | OpenPanorama Panorama File |
| opq | Eschalon: Book I Game Data |
| oprc | Palm OS Desktop OPRC Data |
| ops | Altair FEKO OPTFEKO Internal Data |
| opt | Concordance Native Viewer Database |
| opt | Altair FEKO CADFEKO Optimisation Data |
| opuscert | Directory Opus Certificate Data |
| opv | OpenPanorama Visit File |
| opy | OptiY Model |
| op_ | GKSetup Support Data |
| or2 | Lotus Organizer 2 Data |
| or3 | Lotus Organizer 97 Data |
| or4 | Lotus Organizer 4 Data |
| or8 | IBM Rational Rose Oracle8 Database Model Report |
| orb | XMVB Optimized VB Orbitals |
| orbx | OctaneRender Scene Package |
| ord | Open Reminder Database |
| 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 |
| os | Altair FEKO POSTFEKO Currents Data |
| os2 | OS/2 Warp System File |
