| obs | ObjectScript Script |
| obs | Objeck Source Code |
| obv | ObjectScript Form |
| oc | Esterel Compiler Automaton Code |
| oca | Visual Basic Control TypeLib Cache |
| ocaml | OCaml Source Code |
| ocb | Origin Code Builder Object |
| occ | CC5X C Compiler Output File |
| ocl | Object Constraint Language File |
| ocml | OCaml Source Code |
| octest | Xcode Objective-C Unit Test Bundle |
| ocz | Origin Encrypted C Source Code |
| odc | OpenDCL File |
| odcl | OpenDCL File |
| odd | TEI Source Code |
| odex | Android Optimized Dalvik Executable |
| odg | Psion OVAL Dialog File |
| odl | Object Description Language |
| odp | OPRO Direct to Print File |
| oem | Windows OEM Setup File |
| ofd | OPRO Form Data |
| ofp | Objecteering Project |
| ofr | Psion OVAL Form |
| ogd | Oracle Forms Graph |
| ogre | OGRE 3D Data |
| ohc | Delphi OpenHelp Contents |
| ohi | Delphi OpenHelp Index |
| ohl | Delphi OpenHelp Links |
| oks | OKScript Script |
| okx | OKScript Script |
| olb | Oracle Forms Object Library Module Binary |
| olb | Synergy/DE Object Library |
| ole | Object Linking and Embedding Data |
| oln | Microsoft Visual C++ Outline Examples Data |
| olt | Oracle Forms Object Library Module Text |