| obs | WAsP Obstacle List |
| obs | AquaCrop Field Observations Data |
| obskin | ObjectBar Skin |
| obu | WindLDR Device Data List |
| obx | pCon.planner Pro OBX Exchange Format Data |
| obx | IBM Rational XDE Object File |
| oc | MODFLOW Output Control Data |
| oc3 | OpenCanvas 3 Event |
| oc4 | OpenCanvas 4 Event |
| oc5 | OpenCanvas 5 Event |
| oc8 | OrgCon 8 Organization Data |
| oc9 | OrgCon 9 Organization Data |
| ocb | DOpus Configuration Backup |
| occ | DB/TextWorks List of BTX Records |
| occ | OptControlCentre Model |
| ocd | OCAD Map |
| ocen | EdGCM OCEN Data |
| ocf | Orbit Construction Data |
| ocf | Object Craft File |
| och | Viewsat FTA Receiver Channel List |
| ocm | AOL Advertising Control Data |
| ocp | Orbit Composite Point Cloud |
| ocr | Orbit Composite Resources Data |
| ocs | OCS Inventory NG Data |
| ocsmeet | Microsoft Lync Online Meeting Data |
| ocsp | HAProxy OCSP Data |
| oct | Radiance Octree |
| ocv | Optical Character Verification Data |
| ocw | Symantec ESM Oracle Configuration Watch Template |
| oc_ | Compressed OCX File |