| 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 |
| AntiX | — | — | GNU General Public License |
| AO-DOS | — | — | — |
| AOS/VS II | Data General | — | — |
| AOSC OS | Anthon Open Source Community | — | — |
| 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. | — | — |
| ArcaOS | Arca Noae | — | end-user license agreement |
| Arch BSD | — | — | — |
| ArchOne | — | — | — |
| Armbian Linux | — | — | GNU General Public License, version 3.0 |
| Armed Forces mobile system | — | — | proprietary license |
| Arthur operating system | Acorn Computers | — | — |
| AskoziaPBX | — | — | — |
| ASRI Edu | — | — | GNU Lesser General Public License, GNU General Public License |
| AstraOS | — | — | — |
| Atari DOS | Atari | — | — |
| Atari System V | Atari | 1991 | — |
| Atari TOS | Atari, Jack Tramiel | — | — |
| AtheOS | Kurt Skauen | — | GNU General Public License |
| AudioOS | Apple Inc. | — | — |
| Augustux | — | — | GNU General Public License |
| Aurora SPARC Linux | — | — | — |
| AVLinux | — | — | — |
| Bada | Samsung Electronics | — | proprietary license |
| Banyan VINES | Banyan Systems | — | — |
| BareMetal | — | — | BSD licenses |
| Barrelfish | — | — | MIT License |
| BeOS | Be Inc., Jean-Louis Gassée, Cyril Meurillon, Jean-Baptiste Maurice Quéru, Benoit Schillings, Dominic Giampaolo, Steve Sakoman | — | proprietary license |
| BeOS R5.1d0 | Be Inc. | — | — |
| Berkeley Timesharing System | University of California, Berkeley | — | — |
| BeRTOS | — | — | GNU General Public License, version 2.0 or later |
| BESYS | Bell Labs | — | — |
| BioSLAX | National University of Singapore | — | — |
| Bitrig | — | — | ISC license |
| BKUNIX | Sergey Vakulenko | — | — |
| Black Lab Linux | — | — | — |
| BlackBerry 10 | BlackBerry | — | proprietary license |