| oct | Radiance Octree |
| ocv | Optical Character Verification Data |
| ocw | Symantec ESM Oracle Configuration Watch Template |
| oc_ | Compressed OCX File |
| od | Mascopt Data |
| od1 | Omnis Studio Database |
| od2 | Omnis Studio Database |
| od3 | Omnis Studio Database |
| od4 | Omnis Studio Database |
| od5 | Omnis Studio Database |
| od6 | Omnis Studio Database |
| od7 | Omnis Studio Database |
| od8 | Omnis Studio Database |
| od9 | Omnis Studio Database |
| oda | AIBO ODA File |
| odat | Hopsan Optimization Data |
| odb | Abaqus/Explicit Output Database |
| odh | Odigo Messenger History Data |
| odif | Open Document Interchange Format Data |
| odn | Fluke Power Analyze Database |
| odt | Origin Dialog Theme Data |
| odu | Odigo Messenger User Data |
| odv | Ocean Data View Data |
| odx | Microsoft BizTalk Server Orchestration File |
| oe3 | OpenCanvas 3 Event Data |
| oe4 | OpenCanvas 4 Event Data |
| oe5 | OpenCanvas 5 Event Data |
| oeaccount | Windows Mail Account Data |
| oeaccount | Windows Mail Account |
| oeb | Outlook Express Backup Wizard Data |
