| Kdmix | — | — | — |
| KERNAL | Commodore International | — | — |
| KeshOS | — | — | — |
| Kid operating system | Thomas Petazzoni | — | — |
| KMON | — | — | — |
| KOBRA | Technische Universität Ilmenau, VEB Robotron | — | — |
| KolibriOS | — | — | GNU General Public License, version 2.0 |
| Kuki Linux | — | — | — |
| Kylin | National University of Defense Technology | — | GNU General Public License |
| L4Linux | — | — | GNU General Public License |
| LainOS | — | — | — |
| LeJOS | — | — | — |
| LeWa OS | — | — | — |
| LG UX | LG Electronics | — | — |
| LibreELEC | — | — | GNU General Public License, version 2.0 |
| Lilith | — | — | MIT License |
| Linux for PlayStation 2 | Sony Interactive Entertainment | — | — |
| Linux Unified Kernel | — | — | GNU General Public License, version 2.0 |
| LinuxBBQ | — | — | — |
| Linuxcare Bootable Toolbox | Linuxcare | — | — |
| Lisa OS | Apple Inc. | — | — |
| Livermore Time Sharing System | Lawrence Livermore National Laboratory | — | — |
| LNX-BBC | — | — | — |
| LOCUS | University of California, Los Angeles | — | proprietary license |
| LOUD | — | — | — |
| LUnix | — | — | GNU General Public License, version 2.0 |
| Mac OS 8 | Apple Inc. | — | proprietary license |
| Mac OS 9 | Apple Inc. | — | proprietary license |
| Mac OS X 10.0 | Apple Inc. | 2001 | — |
| Mac OS X 10.1 | Apple Inc. | 2001 | Apple Public Source License |
| Mac OS X 10.2 | Apple Inc. | 2002 | Apple Public Source License, end-user license agreement |
| Mac OS X Leopard | Apple Inc. | 2007 | Apple Public Source License, end-user license agreement |
| Mac OS X Lion | Apple Inc. | 2011 | Apple Public Source License |
| Mac OS X Panther | Apple Inc. | 2003 | — |
| Mac OS X Public Beta | Apple Inc. | — | — |
| Mac OS X Server 1.0 | Apple Inc. | — | — |
| Mac OS X Snow Leopard | Apple Inc. | 2009 | Apple Public Source License |
| Mac OS X Tiger | Apple Inc. | 2005 | — |
| MachTen | — | — | — |
| MacMach | Carnegie Mellon University | — | — |
| MacOS Big Sur | Apple Inc. | 2020 | proprietary license, Apple Public Source License |
| MacOS Catalina | Apple Inc. | 2019 | Apple Public Source License |
| MacOS High Sierra | Apple Inc. | 2017 | Apple Public Source License |
| MacOS Mojave | Apple Inc. | 2018 | Apple Public Source License |
| MacOS Monterey | Apple Inc. | 2021 | proprietary license, Apple Public Source License |
| MacOS Sequoia | Apple Inc. | — | — |
| MacOS Server | Apple Inc. | 1999 | proprietary license, commercial software |
| MacOS Sierra | Apple Inc. | 2016 | Apple Public Source License |