| 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 |
| B2G OS | — | — | Mozilla Public License |
| BackBox | — | — | GNU General Public License |
| BackSlash Linux | Kumar Priyansh | — | Apache Software License 2.0 |
| BackTrack | — | 2006 | GNU General Public License |
| Bada | Samsung Electronics | — | proprietary license |
| Baidu Yi | Baidu | — | — |
| Baltix | — | — | GNU General Public License |
| Bankix | — | — | — |
| Banyan VINES | Banyan Systems | — | — |
| BareMetal | — | — | BSD licenses |
| Barrelfish | — | — | MIT License |
| BasicLinux | — | — | — |
| Batocera.linux | — | — | GNU General Public License |
| Bazzite | — | — | Apache Software License 2.0 |
| BeatrIX | — | — | — |
| Bedrock Linux | — | — | GNU General Public License, version 2.0 |
| Behördendesktop | Federal Office for Information Security | — | — |
| BeleniX | — | — | Common Development and Distribution License version 1.0 |
| BengalOS | — | — | — |
| 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 Software Distribution | University of California, Berkeley | — | BSD licenses |
| Berkeley Timesharing System | University of California, Berkeley | — | — |
| Berry Linux | — | — | — |
| BeRTOS | — | — | GNU General Public License, version 2.0 or later |
| BESYS | Bell Labs | — | — |
| Bharat Operating System Solutions | Centre for Development of Advanced Computing | — | GNU General Public License |