| omd | Routing J Server Data |
| omf | Routing J Server Routing Data |
| omm | OptiMatrix Maker Data |
| oms | Order Maven Data |
| oms | LightShow Pro Optimized Media Sequence |
| omt | HLA Object Model Template |
| omx | OMAX Routed Data File |
| ond | Lotus Notes Database |
| onebin | Microsoft OneNote Binary Data |
| onepkg | Microsoft OneNote Package |
| onetmp | Microsoft OneNote Temporary File |
| onetoc | Microsoft OneNote Table of Contents |
| onetoc2 | Microsoft OneNote Table of Contents |
| onl | PATCHMASTER Online Analysis Information |
| onm | Outdoor Navigator Map |
| ont | theWord Bible Text Module |
| ontx | theWord Encrypted Bible Text Module |
| onw | Omron NTST Data |
| onx | ONYX PosterShop Packaged Job |
| ooa | OOA Interchange Format Data |
| oob | Sybase PowerDesigner Object-oriented Model Backup |
| ooc | CHARTrunner Out-of-control Tests Definition |
| ook | Arachne Batch Script |
| ool | Orbit GIS Object List |
| oom | Sybase PowerDesigner Object-oriented Model |
| ooxml | Office Open XML File |
| op2 | Nastran Binary Results |
| op4 | Nastran Binary Results |
| opal | Microsoft Office Language Specific Resources |
| opc | Office Upgrade Control |
