| O:XML | object-oriented programming | — | — |
| Oak | object-oriented programming | 1989 | James Gosling |
| Oaklisp | object-oriented programming | 1986 | — |
| Oberon | procedural programming, structured programming, imperative programming, modular programming, object-oriented programming | 1987 | Niklaus Wirth |
| Oberon-2 | imperative programming, object-oriented programming | 1991 | Niklaus Wirth |
| Obix | — | — | — |
| Object Lisp | prototype-based programming, object-oriented programming | — | — |
| Object Oberon | object-oriented programming | 1990 | — |
| Object Pascal | generic programming, procedural programming, structured programming, imperative programming, multi-paradigm programming, object-oriented programming | 1986 | Larry Tesler, Niklaus Wirth, Apple Inc., Borland |
| Object REXX | object-oriented programming | 1988 | IBM |
| Objective-C | object-oriented programming, class-based programming, multi-paradigm programming, reflective programming | 1984 | — |
| Objective-J | reflective programming, functional programming, imperative programming, object-oriented programming | 2008 | — |
| ObjVlisp | — | 1984 | — |
| ObjVProlog | — | 1989 | — |
| Obliq | object-oriented programming, prototype-based programming, structured programming, parallel computing, imperative programming, modular programming | 1993 | — |
| OCaml | functional programming, modular programming, object-oriented programming, imperative programming | 1996 | Institut National de Recherche en Informatique et en Automatique |
| Occam | imperative programming, procedural programming, concurrent computing | 1983 | Inmos |
| Occam-π | procedural programming, imperative programming, concurrent computing | 2006 | — |
| Octave | — | — | — |
| Odin | — | 2016 | — |
| OMNIBAC Symbolic Assembler | — | 1951 | — |
| ONScripter | — | — | — |
| Ooc | — | — | — |
| Opa | imperative programming, multi-paradigm programming, functional programming | 2011 | — |
| Opal | functional programming | — | — |
| Open Programming Language | imperative programming, structured programming | 1984 | — |
| Open Roberta | event-driven programming | — | — |
| Open Shading Language | — | — | Sony Pictures Imageworks, Larry Israel Gritz |
| OpenCL | object-oriented programming, imperative programming, structured programming, generic programming | 2009 | Khronos Group |
| OpenEdge Advanced Business Language | object-oriented programming | — | Progress Software |
| OpenGL ES Shading Language | — | 2009 | Khronos Group |
| OpenGL Shading Language | — | 2004 | OpenGL Architecture Review Board |
| OpenJDK | — | 2007 | OpenJDK Community, Oracle Corporation, Sun Microsystems |
| OpenLisp | — | — | — |
| Openmodelica | declarative programming | — | — |
| OpenMusic | — | — | — |
| OpenMx | structural equation modeling | 2010 | — |
| OpenQASM | — | — | IBM Research |
| OPS5 | functional programming | — | — |
| OptimJ | object-oriented programming | 2006 | — |
| Orc | concurrent computing | 2004 | Jayadev Misra, David Kitchin, William Cook |
| Orca | — | — | Hundred Rabbits |
| ORCA/Modula-2 | — | 1994 | The Byte Works |
| Oriel | procedural programming, imperative programming | 1991 | — |
| Orwell | functional programming | 1984 | — |
| Otus Lisp | — | — | — |
| OTX | — | 2012 | — |
| Overpass QL | imperative programming, procedural programming | — | Roland Olbricht |
| Overpass XML | imperative programming, procedural programming | — | Roland Olbricht |
| Ox | — | — | — |
| Oxygene | parallel computing, object-oriented programming | 2005 | RemObjects Software |
| Oz | object-oriented programming, constraint programming, concurrent computing, prototype-based programming, imperative programming, functional programming, multi-paradigm programming | 1991 | — |