| A/ROSE | Apple Inc. | — | — |
| A/UX | Apple Inc. | — | proprietary license |
| A2 | ETH Zurich, Niklaus Wirth | — | BSD licenses |
| ABC GNU/Linux | — | — | — |
| ABCbuntu | — | — | — |
| Abitti | The Matriculation Examination Board of Finland | — | — |
| Absolute Linux | — | — | GNU General Public License |
| AbulÉdu Live | — | — | GNU General Public License |
| Acorn MOS | Acorn Computers | — | — |
| ACOS-4 | NEC | — | — |
| ACOS-6 | NEC | — | — |
| Adamantix | — | — | GNU General Public License |
| AdiOS | AdiOS | — | proprietary license |
| AdJ | — | — | — |
| Advanced Comprehensive Operating System | NEC | — | — |
| Advanced XPerience OS | — | — | — |
| Adélie Linux | — | — | — |
| AerOS | — | — | — |
| AerynOS | — | — | — |
| Ageless Linux | — | — | — |
| AgiliaLinux | — | — | — |
| AICP | — | — | — |
| Akaros | — | — | GNU General Public License, version 2.0 |
| Alinex | — | — | — |
| ALinux | — | — | — |
| AliOS | Alibaba Group | — | — |
| AlliedWare Plus | Allied Telesis Holdings | — | — |
| AlmaLinux | Andrew Lukoshko | — | — |
| AlphaMicro Operating System | Alpha Microsystems | — | — |
| Alpine Linux | — | — | 2-clause BSD License, GNU General Public License, version 2.0, MIT License |
| ALT Linux | ALT Linux Team, ALT Linux | — | GNU General Public License, version 2.0 or later |
| Alter Linux | — | — | — |
| Altimit OS | — | — | — |
| AmayaOS | — | 2014 | GNU General Public License, version 3.0 |
| Amazon Linux | Amazon | 2011 | — |
| Amber Linux | — | — | — |
| AMIBIOS | American Megatrends | 1985 | proprietary license |
| Amiga Unix | Commodore International | — | — |
| AmigaDOS | Commodore International | — | proprietary license |
| AmigaOS | Hyperion Entertainment, Haage & Partner, Commodore International | — | end-user license agreement |
| AmigaOS 4 | Hyperion Entertainment | 2006 | proprietary license |
| AmiKit | — | — | proprietary license |
| Amoeba | Andrew S. Tanenbaum | — | MIT License |
| AmogOS | — | — | 4-clause BSD License |
| AmonEcole | EOLE | — | — |
| AMSDOS | Amstrad | — | — |
| Analogue OS | Analogue Inc. | — | — |
| ANDOS | — | — | — |
| Android | Google | — | Apache Software License 2.0, GNU General Public License, version 2.0 |
| Android 1.0 | — | — | Apache Software License 2.0 |
| Android 1.1 | — | 2009 | — |
| Android 10 | Google, Open Handset Alliance | 2019 | Apache License |
| Android 11 | Open Handset Alliance | — | — |
| Android 12 | Google | — | proprietary license |
| Android 13 | Open Handset Alliance, Google | — | — |
| Android 14 | Google | — | — |
| Android 15 | Google | — | — |
| Android 16 | Google, Open Handset Alliance | — | — |
| Android 17 | Android Jetpack, Open Handset Alliance | — | — |
| Android Auto | Google | 2015 | proprietary license |
| Android Automotive | — | 2017 | — |
| Android Canary | — | 2025 | — |
| Android Cupcake | Google | — | — |
| Android Donut | Google | — | — |
| Android Eclair | Google | — | — |
| Android Froyo | Google | — | — |
| Android Gingerbread | Google | 2010 | — |
| Android Go | — | 2017 | — |
| Android Honeycomb | Google | — | — |
| Android Ice Cream Sandwich | Google | — | — |
| Android Jelly Bean | Google | — | — |
| Android KitKat | Open Handset Alliance, Google | — | Apache License, GNU General Public License |
| Android Lollipop | — | 2014 | Apache Software License 2.0, Android Open Source Project, GNU General Public License, version 2.0 |
| Android Marshmallow | Open Handset Alliance, Google | — | free software, Android Open Source Project |
| Android Nougat | Google, Open Handset Alliance | 2016 | Android Open Source Project |
| Android on Raspberry Pi | — | — | — |
| Android Open Source Project | — | — | Creative Commons Attribution 2.5 Generic, GNU General Public License, Apache License |
| Android Oreo | Google, Open Handset Alliance | 2017 | Android Open Source Project, Apache License |
| Android Pie | Google, Open Handset Alliance | 2018 | Apache License |
| Android Things | — | — | Apache License |
| Android TV | Google | 2014 | proprietary license |
| Android XR | Google, Samsung Electronics | — | — |
| Android-x86 | — | 2017 | Apache License |
| AnduinOS | — | — | GNU General Public License, version 3.0 |
| Annvix | — | — | — |
| Antergos | — | 2013 | GNU General Public License |
| Anthon OS | — | — | — |
| AntiX | — | — | GNU General Public License |
| AO-DOS | — | — | — |
| AOKP | George Merlocco | — | Apache License |
| AOS/VS II | Data General | — | — |
| AOSC OS | Anthon Open Source Community | — | — |
| AospExtended | — | 2017 | — |
| Apache Mynewt OS | Apache Software Foundation | — | — |
| Apple CP/M | Apple Inc. | — | — |
| Apple DOS | Apple Inc. | 1978 | — |
| Apple GS/OS | Apple Inc. | — | — |
| Apple Pascal | Apple Inc. | — | — |
| Apple ProDOS | Apple Inc. | — | — |
| Apple SOS | Apple Inc. | — | — |
| Apricity OS | Alex Gajewski | — | — |
| Aptosid | — | 2006 | GNU General Public License |
| ArcaOS | Arca Noae | — | end-user license agreement |
| Arch BSD | — | — | — |
| Arch Hurd | — | 2011 | GNU General Public License |
| Arch Linux | Allan McRae, Anatol Pomozov, Aaron Griffin, Felix Yan | 2002 | GNU General Public License |
| Arch Linux 32 | — | — | — |
| Arch Linux ARM | — | — | — |
| ArchBang | — | 2010 | GNU General Public License |
| Archcraft | — | — | — |
| Archie Linux | — | — | — |
| ArchLabs | — | — | — |
| ArchOne | — | — | — |
| ArcoLinux | — | — | — |
| Ark Linux | — | 2005 | — |
| ArkOS | — | — | MIT License |
| ArkOS | — | — | GNU General Public License |
| ARMA aka Omoikane GNU/Linux | — | — | — |
| Armbian Linux | — | — | GNU General Public License, version 3.0 |
| Armed Forces mobile system | — | — | proprietary license |
| AROS Research Operating System | — | — | AROS Public License |
| Arthur operating system | Acorn Computers | — | — |
| ArtistX | — | — | — |
| Artix Linux | — | 2017 | — |
| ARX | Acorn Computers | — | — |
| Asahi Linux | Asahi Lina, Alyssa Rosenzweig, Hector Martin | 2021 | — |
| Asianux | — | — | — |
| AskoziaPBX | — | — | — |
| ASLinux Desktop | — | — | — |
| ASRI Edu | — | — | GNU Lesser General Public License, GNU General Public License |
| AsteroidOS | — | — | — |
| Astra Linux | RusBITech | — | GNU General Public License |
| AstraOS | — | — | — |
| Asturix | — | — | GNU General Public License |
| ASUS Express Gate | ASUS | — | — |
| Atari DOS | Atari | — | — |
| Atari System V | Atari | 1991 | — |
| Atari TOS | Atari, Jack Tramiel | — | — |
| Athena OS | — | — | MIT License |
| AtheOS | Kurt Skauen | — | GNU General Public License |
| ATmission | — | — | — |
| Atomic-OS | — | 2017 | — |
| AudioOS | Apple Inc. | — | — |
| Auditor | — | — | — |
| Augustux | — | — | GNU General Public License |
| Aurora | — | — | GNU General Public License |
| Aurora OS | — | — | — |
| Aurora SPARC Linux | — | — | — |
| AUSTRUMI Linux | Andrejs Meinerts | — | GNU General Public License |
| AVLinux | — | — | — |
| Award BIOS | Award Software, Phoenix Technologies | — | — |
| Axle OS | — | — | MIT License |
| AxOS | — | — | GNU General Public License, version 3.0 |
| Axyl OS | — | — | — |
| Azure Linux | Microsoft | — | MIT License, GNU General Public License |
| Azure Sphere OS | Microsoft | — | MIT License |
| Ångström distribution | — | — | GNU General Public License |