| M0n0wall | — | — | BSD licenses |
| Mabox Linux | — | — | — |
| 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 | — |
| MacDroid | — | — | — |
| MachTen | — | — | — |
| MacMach | Carnegie Mellon University | — | — |
| MacOS | Apple Inc. | 2001 | BSD licenses, proprietary license, Apple Public Source License |
| MacOS Big Sur | Apple Inc. | 2020 | proprietary license, Apple Public Source License |
| MacOS Catalina | Apple Inc. | 2019 | Apple Public Source License |
| MacOS Golden Gate | Apple Inc. | 2026 | — |
| 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 |
| MacOS Sonoma | Apple Inc. | — | — |
| MacOS Tahoe | — | — | — |
| MacOS Ventura | Apple Inc. | 2022 | — |
| Macpup | — | — | GNU General Public License |
| Madbox Linux | — | — | — |
| Maemo | Nokia | — | — |
| Maemo Leste | — | — | — |
| Mageia | — | — | GNU General Public License |
| Magic Cap | General Magic | 1994 | — |
| Magic Linux | — | — | — |
| Magic UI | Honor | — | — |
| MagicOS | Honor | 2016 | proprietary license |
| MagicOS 10 | Honor | — | — |
| MagicOS 9 | Honor | — | — |
| MagicSix | — | — | — |
| MagpieOS | — | — | — |
| MAid Linux | — | — | — |
| MakuluLinux | — | — | — |
| Mandriva Linux | Mandriva | — | GNU General Public License |
| Mangaka Linux | — | — | — |
| Manjaro | Philip Müller | 2011 | free software |
| MaRTE OS | University of Cantabria | — | GNU General Public License, version 2.0 |
| MASTAR | STMicroelectronics | — | — |
| Mastercam | — | — | — |
| Maui Linux | — | — | — |
| Mayix XliveCD | — | — | — |
| MCC Interim Linux | — | — | — |
| McKernel | RIKEN | — | GNU General Public License, version 2.0 |
| MCNLive | — | — | — |
| Mcube Kernel | — | — | 2-clause BSD License |
| MDOS | — | — | — |
| MEDINA | T-Systems International GmbH | — | — |
| MeeGo | Linux Foundation, Intel, Novell, Nokia, AMD | — | GNU General Public License, version 2.0 or later |
| MeNTOPPIX | — | — | — |
| MenuetOS | — | — | proprietary license, GNU General Public License, version 2.0 |
| MEPIS | Warren Woodford | — | GNU General Public License |
| Meta Horizon OS | Meta Platforms | — | — |
| METALIVE | Dream cast, PRIME HOLDINGS | — | — |
| Mezzano | — | — | MIT License |
| Michigan Terminal System | University of Michigan | 1967 | — |
| Micro T-Kernel | T-Engine Forum | — | — |
| MicroBSD | — | — | BSD licenses |
| MicroC/OS-III | — | — | — |
| MicroDOS | — | — | — |
| MicroOS | — | — | — |
| Microsoft Forefront | — | — | — |
| Microsoft Windows | Microsoft | 1985 | proprietary license |
| Microsoft Windows 98 Second Edition | Microsoft | — | — |
| MIDAS | Microsoft | — | — |
| MIDIbox Operating System | — | — | — |
| MidnightBSD | — | 2007 | BSD licenses |
| Midori | Microsoft | — | — |
| MikeOS | — | — | BSD licenses |
| Mikros | — | — | — |
| MilaX | — | — | Common Development and Distribution License |
| MineHarmony OS | Huawei, CHN Energy | — | — |
| Minimal Linux Live | — | — | — |
| MiniOS | — | — | — |
| MINIX | Andrew S. Tanenbaum | — | 3-clause BSD License |
| MINIX 3 | Andrew S. Tanenbaum | — | BSD licenses |
| Minoca OS | — | — | GNU General Public License, version 3.0 |
| MiNT | — | — | — |
| MintPPC | — | — | — |
| Miosix | Federico Terraneo | 2009 | GNU General Public License |
| MIPS RISC/os | MIPS Technologies | — | — |
| Miracle Linux | Cybertrust | — | — |
| Miraculix | — | — | freeware |
| MirageOS | Thomas Gazagnaire | — | ISC license |
| MirOS BSD | — | — | BSD licenses |
| Miru directory server | — | — | — |
| MIUI | Xiaomi | — | GNU General Public License, version 3.0, Apache Software License 2.0 |
| MkLinux | Free software movement, Open Software Foundation | — | GNU General Public License |
| MNOS | — | — | — |
| Mobian | — | — | — |
| Mobilinux | MontaVista | 2005 | — |
| Moblin | Linux Foundation | — | — |
| MocaccinoOS | — | — | — |
| MoKee ROM | — | 2012 | MIT License, GNU General Public License, version 2.0 |
| Molinux | — | — | GNU General Public License |
| Momonga Linux | — | — | — |
| MonaOS | — | — | MIT License |
| MONECS | Monash University | — | — |
| Monoppix | — | — | — |
| MontaVista CGX Linux | MontaVista | — | — |
| Morphix | — | — | GNU General Public License |
| MorphOS | — | 2000 | — |
| MOS | — | — | — |
| Motoblur | Motorola Mobility | — | — |
| MotoMagx | Motorola | — | — |
| MoviX | — | — | GNU General Public License |
| MP/M | Digital Research, Tom Rolander | — | proprietary license |
| MQX | — | — | — |
| MS-DOS | Microsoft | 1981 | MIT License, proprietary license |
| MS-DOS 1.25 | Microsoft | 1982 | MIT License |
| MS-DOS 2.0 | Microsoft | 1983 | MIT License |
| MS-DOS 4.0 | Microsoft | 1988 | MIT License |
| MS-DOS 6.22 | Microsoft | 1994 | proprietary license |
| MS-DOS 7 | Microsoft | — | proprietary license |
| MS/8 | Richard Lary | — | — |
| MSX-DOS | Microsoft | — | — |
| Mubuntu | — | — | — |
| MuLinux | — | — | GNU General Public License |
| Multi-Environment Real-Time | Bell Labs | — | — |
| Multi-tasking MS-DOS 4.0 | Microsoft | 1986 | MIT License |
| Multics | Bell Labs, Massachusetts Institute of Technology, General Electric | 1969 | MIT License |
| Multiple Console Time Sharing System | General Motors Research Laboratories | — | — |
| Multiuser DOS | Digital Research | — | — |
| MULTOS | — | — | — |
| MUSIC/SP | McGill University | 1972 | — |
| Musix GNU/Linux | — | — | — |
| MVS | IBM | 1974 | proprietary license |
| MX Linux | — | — | GNU General Public License |
| MyLinux | — | — | — |
| Mythbuntu | — | — | GNU General Public License |
| MythDora | — | — | — |
| Möbius | — | — | GNU General Public License |