| NX-OS | Cisco | — | — |
| OASIS operating system | — | — | — |
| Oberon | Niklaus Wirth | — | BSD licenses |
| Odebian | — | — | — |
| Olivetti X/OS | Olivetti | — | — |
| One UI Core | — | — | — |
| Online OS | — | — | European Union Public Licence |
| ONTAP | NetApp | — | — |
| Open Enterprise Server | Novell | — | — |
| Open-E DSS | Open-E | — | — |
| OpenComRTOS | — | — | — |
| OpenDOS | Caldera | — | — |
| OpenGEU | — | — | GNU General Public License |
| OpenHarmony | Huawei, OpenAtom Foundation | 2020 | Apache Software License 2.0 |
| OpenKylin | — | — | — |
| OpenMosix | — | — | — |
| OpenSolaris for System z | — | — | Common Development and Distribution License |
| OPENSTEP | — | — | — |
| OpenSUSE MicroOS | — | — | — |
| OpenTag | — | — | — |
| OpenVMS | VMS Software Inc., Hewlett-Packard, Digital Equipment Corporation, Compaq | 1977 | proprietary license |
| Operating System Embedded | ENEA AB | — | — |
| OPhone | — | — | Apache License |
| OpuntiaOS | — | — | 2-clause BSD License |
| Origin OS | Vivo | — | — |
| ORVYL and WYLBUR | Stanford University | — | — |
| Oryx/Pecos | Bell Labs | — | — |
| OS 2200 | Unisys | — | — |
| OS 703 | TRS Zagreb | — | — |
| OS 901 | TRS Zagreb | — | — |
| OS ES | — | — | — |
| OS X El Capitan | Apple Inc. | 2015 | Apple Public Source License |
| OS X Mavericks | Apple Inc. | 2013 | Apple Public Source License |
| OS X Mountain Lion | Apple Inc. | 2012 | Apple Public Source License |
| OS X Yosemite | Apple Inc. | 2014 | Apple Public Source License |
| OS-9 | Microware | — | — |
| Os.bf | Brainfuck Enterpise Solutions | — | WTFPL |
| OS/2 | IBM, Microsoft | — | proprietary license |
| OS/2 Warp 4 | IBM | 1996 | — |
| OS/2 Warp Server | IBM | — | — |
| OS/360 | IBM | 1966 | proprietary license |
| OS/390 | IBM | 1995 | — |
| OS/400 | IBM | — | — |
| OS/8 | Digital Equipment Corporation | — | — |
| OS/VS1 | IBM | — | — |
| OS/VS2 | IBM | 1972 | — |
| OS2000 | Scientific Research Institute of System Development | — | — |
| OS4000 | GEC Computers | 1977 | — |