| nxc | Not eXactly C Source Code |
| nxd | AdaOS Debugging Information |
| nxg | eSite Builder NXG Web Page |
| nxu | C4 Engine XML Model Data |
| o | Compiled Object File |
| oat | Android Compiled OAT |
| ob2 | Oberon 2 Source Code |
| obak | WinSQL Offline Backup Data |
| obe | Objeck Executable File |
| obh | Hitachi Ladder Editor Object Program |
| obj | Compiled Object File |
| obl | Objeck Compiled Library |
| obn | Oberon 2 Module Source Code |
| obr | Borland C++ Object Browser Data |
| 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 |