| op4 | Nastran Binary Results |
| opal | Microsoft Office Language Specific Resources |
| opb | Gurobi Optimizer Pseudo-boolean Optimization Data |
| opc | Office Upgrade Control |
| opcses | Opticodec-PC Session Data |
| opd | Wyko NT9100 Surface Profiler Surface Data |
| opdownload | Opera Partially Downloaded File |
| ope | IRCS Observation Procedure Execution Data |
| openbus | Altium Designer OpenBus System Document |
| opi | Adlib Express OCR Processing Information |
| opi | OpenPanorama Image File |
| opj | OmniPage Job Data |
| opl | OpenPanorama Lensflare File |
| opm | Quadrox Digital CCTV System Components Data |
| opop | SOCSIM Output Population File |
| opox | SOCSIM Output Extra Variables File |
| opp | OpenPanorama Panorama File |
| opt | Concordance Native Viewer Database |
| opuscert | Directory Opus Certificate Data |
| opv | OpenPanorama Visit File |
| opy | OptiY Model |
| 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 | 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 |