| Canaima | CANTV, CNTI | — | GNU General Public License |
| CAOS Linux | — | 2004 | — |
| CapROS | — | — | GNU General Public License |
| CarbonROM | — | — | — |
| Cassette Aided Operation System | Kombinat Mikroelektronik Erfurt | 1984 | — |
| CB UNIX | Bell Labs | — | — |
| CD-RTOS | — | — | — |
| CDC Kronos | Control Data Corporation | — | — |
| CDC SCOPE | Control Data Corporation | — | — |
| Cdlinux.pl | — | 2001 | GNU General Public License |
| CDOS | — | — | — |
| Celerra | Dell EMC | — | — |
| CentOS | — | 2004 | GNU General Public License |
| CentOS Stream | Red Hat, IBM | 2019 | GNU General Public License |
| Chakra Linux | — | — | GNU General Public License |
| ChaletOS | — | — | — |
| CHAOS | — | — | — |
| Chapeau | — | — | — |
| Cherry OS | Cherry | — | — |
| ChibiOS/RT | — | — | Apache Software License 2.0, GNU General Public License, version 3.0 |
| Chimera Linux | — | — | — |
| ChimeraOS | — | — | — |
| 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 |
| ChromeOS | Google | — | proprietary license |
| ChromeOS Flex | Neverware | 2015 | — |
| ChromiumOS | Google | 2009 | BSD licenses |
| Cinux | — | — | — |
| CircusOS | Circus SE | — | — |
| Cisco IOS | Cisco | — | — |
| 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. | — | — |
| Clear Linux OS | Intel | — | — |
| ClearOS | — | — | GNU General Public License |
| Clockwork OS | ClockworkPi | — | — |
| Clonezilla Live | — | — | — |
| Cloud OS | — | — | Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported |
| CloudLinux | — | — | — |
| Cobalt Qube | Cobalt Networks | 1998 | — |
| Cobuntu | — | — | — |
| Coherent | Mark Williams Company | — | 3-clause BSD License |
| Colebuntu | — | — | — |
| Collapse OS | — | — | GNU General Public License, version 3.0 |
| College Linux | — | — | — |
| ColorOS | Oppo | — | — |
| ColtOS | — | — | — |