| olif | Open Lexicon Interchange Format |
| olk | Microsoft Outlook Address Book |
| olk14category | Microsoft Office Outlook for Mac Category Data |
| olk14contact | Microsoft Office Outlook for Mac Contacts Data |
| olk14event | Microsoft Office Outlook for Mac Events Data |
| olk14group | Microsoft Office Outlook for Mac Group Data |
| olk14message | Microsoft Office Outlook for Mac Message Pointer |
| olk14msgsource | Microsoft Office Outlook for Mac Message Data |
| olk14note | Microsoft Office Outlook for Mac Note |
| olk14signature | Microsoft Office Outlook for Mac Signature |
| olk14task | Microsoft Office Outlook for Mac Task Data |
| olk15event | Microsoft Office Outlook for Mac Events Data |
| oll | TrialDirector Batch Load Data |
| olm | Microsoft Office Outlook for Mac Database |
| ols | WinOLS OLS Data |
| olx | Open Learning XML Data |
| oly | Act! Opportunities Layout |
| oly | AFP Overlay Resource |
| om | STK Owner/Mission File |
| omar | SOCSIM Output Marriage File |
| omb | Omnibus Offline Web Storage Data |
| omcs | Office Manager SQL Archive |
| omd | Routing J Server Data |
| omf | Open Media Framework file |
| omf | Routing J Server Routing Data |
| omfi | Open Media Framework file |
| omlog | CA OM Web Viewer Log |
| omm | OptiMatrix Maker Data |
| omn | Omnipotent Collection Text |
| omr | OMR Reader Template |
| oms | Order Maven Data |
| oms | LightShow Pro Optimized Media Sequence |
| omt | HLA Object Model Template |
| omx | OMAX Routed Data File |
| omx | hyperMILL CAM Temporary Calculation Data |