| BlackBerry OS | BlackBerry | 1999 | proprietary license |
| BlackBerry Tablet OS | BlackBerry | — | — |
| BLIS/COBOL | — | — | — |
| BluOS | NAD Electronics | — | — |
| BOS/360 | IBM | — | proprietary license |
| BrickOS | — | — | — |
| BS2000 | Fujitsu Technology Solutions | 1975 | — |
| BSD | — | — | — |
| BSD Router Project | — | — | BSD licenses |
| BSD/OS | Berkeley Software Design | — | — |
| Buenos | — | — | — |
| Bullx SCS | Atos SE | — | — |
| Burroughs MCP | Burroughs Corporation | 1961 | — |
| CAINE Linux | — | — | — |
| Cairo | Microsoft | — | end-user license agreement |
| Caldera OpenLinux | Caldera | — | — |
| Cambridge Distributed Computing System | University of Cambridge Department of Computer Science and Technology | — | — |
| CapROS | — | — | GNU General Public License |
| Cassette Aided Operation System | Kombinat Mikroelektronik Erfurt | 1984 | — |
| CB UNIX | Bell Labs | — | — |
| CD-RTOS | — | — | — |
| CDC Kronos | Control Data Corporation | — | — |
| CDC SCOPE | Control Data Corporation | — | — |
| CDOS | — | — | — |
| Celerra | Dell EMC | — | — |
| Cherry OS | Cherry | — | — |
| ChibiOS/RT | — | — | Apache Software License 2.0, GNU General Public License, version 3.0 |
| China Operating System | — | — | — |
| ChorusOS | Chorus Systèmes, Oracle Corporation, Sun Microsystems, Institut National de Recherche en Informatique et en Automatique | — | BSD licenses, GNU Lesser General Public License, Mozilla Public License, Sun Public License |
| CircusOS | Circus SE | — | — |
| Cisco IOS XE | Cisco | — | proprietary license |
| Cisco Security Monitoring, Analysis, and Response System | Cisco | — | — |
| Citrix WinFrame | Citrix Systems | 1995 | proprietary license |
| Classic Mac OS | Apple Inc. | — | — |
| Cloud OS | — | — | Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported |
| Cobalt Qube | Cobalt Networks | 1998 | — |
| Coherent | Mark Williams Company | — | 3-clause BSD License |
| Commotion | — | — | — |
| Compatible Time-Sharing System | Massachusetts Institute of Technology | — | — |
| Computer System Operating System | IBM | — | — |
| ContactOffice | — | — | — |
| Contiki-NG | George Oikonomou, Simon Duquennoy | — | 3-clause BSD License |
| Convergent Technologies Operating System | Unisys | — | — |
| Conversational Monitor System | — | — | — |
| Cooperation-iws | — | — | — |
| Copland | Apple Inc. | — | — |
| CoreELEC | — | — | GNU General Public License, version 2.0 |
| COSIX | — | — | — |