| BharOS | Indian Institute of Technology Madras | — | — |
| Bifrost | — | — | — |
| Big Linux | Bruno Gonçalves | — | — |
| Binary Runtime Environment for Wireless | Qualcomm | — | — |
| BinToo GNU / Linux | — | — | — |
| BioSLAX | National University of Singapore | — | — |
| Bitrig | — | — | ISC license |
| BKUNIX | Sergey Vakulenko | — | — |
| Black Lab Linux | — | — | — |
| BlackArch Linux | — | — | 3-clause BSD License |
| BlackBerry 10 | BlackBerry | — | proprietary license |
| BlackBerry OS | BlackBerry | 1999 | proprietary license |
| BlackBerry Tablet OS | BlackBerry | — | — |
| BlackPanther OS | — | — | — |
| BLAG Linux and GNU | — | — | GNU General Public License |
| BlankOn | — | — | — |
| BlendOS | — | — | — |
| BLIS/COBOL | — | — | — |
| Bliss OS | — | — | GNU General Public License, version 3.0 |
| Bluefin | — | — | — |
| Bluewhite64 | — | — | — |
| BluOS | NAD Electronics | — | — |
| Bodhi Linux | — | — | GNU General Public License |
| Bonzai Linux | — | — | — |
| Boot-root | H J Lu | — | — |
| BOS/360 | IBM | — | proprietary license |
| BrickOS | — | — | — |
| Bridge Linux | — | — | GNU General Public License |
| BridgeOS | Apple Inc. | — | proprietary license |
| BRTOS | — | — | — |
| BS2000 | Fujitsu Technology Solutions | 1975 | — |
| BSD | — | — | — |
| BSD Router Project | — | — | BSD licenses |
| BSD/OS | Berkeley Software Design | — | — |
| Buenos | — | — | — |
| Buildix | ThoughtWorks | — | — |
| Bullx SCS | Atos SE | — | — |
| BunsenLabs | — | 2015 | — |
| Burroughs MCP | Burroughs Corporation | 1961 | — |
| CachyOS | — | — | GNU General Public License |
| CAElinux | — | — | — |
| CAINE Linux | — | — | — |
| Cairo | Microsoft | — | end-user license agreement |
| Caixa Mágica | — | — | — |
| Calculate Linux | — | — | GNU General Public License |
| Caldera OpenLinux | Caldera | — | — |
| CalyxOS | Calyx Institute | 2020 | — |
| Cambridge Distributed Computing System | University of Cambridge Department of Computer Science and Technology | — | — |