| 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 | — | — |
| 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 | — | — | — |
| Commodore DOS | Commodore International | — | — |
| Commodore OS | Commodore USA | 2010 | GNU General Public License |
| Commotion | — | — | — |
| Compatible Time-Sharing System | Massachusetts Institute of Technology | — | — |
| Compute Node Linux | Cray | — | — |
| Computer System Operating System | IBM | — | — |
| Condres OS | — | — | — |
| Conectiva | — | — | — |
| ConnochaetOS | Henry Jensen | — | — |
| ContactOffice | — | — | — |
| Container Linux | — | — | Apache License |
| Contiki | Adam Dunkels | — | 3-clause BSD License |
| Contiki-NG | George Oikonomou, Simon Duquennoy | — | 3-clause BSD License |
| Convergent Technologies Operating System | Unisys | — | — |
| Conversational Monitor System | — | — | — |
| Cooperation-iws | — | — | — |
| Copland | Apple Inc. | — | — |
| CopperheadOS | — | 2015 | Creative Commons |
| CoreELEC | — | — | GNU General Public License, version 2.0 |
| Corel Linux | Corel | — | GNU General Public License |
| COSIX | — | — | — |
| Cosmoe | Bill Hayden | — | GNU General Public License |
| Cosmos | — | — | BSD licenses |
| Cosmos (Olivetti) | Olivetti | — | — |
| Coyote Linux | — | — | GNU General Public License |
| Coyotos | — | — | — |
| CP-67 | Cambridge Scientific Center | — | — |
| CP/CMS | Cambridge Scientific Center | — | — |
| CP/J | — | — | — |
| CP/M | Digital Research, Gary Kildall | — | BSD licenses |
| CP/M-86 | Digital Research | — | proprietary license |
| Cp6linux | — | — | — |
| Cray Operating System | Cray | — | — |
| Cray Time Sharing System | Lawrence Livermore National Laboratory, Los Alamos National Laboratory | — | — |
| CrDroid | — | — | — |
| CROWZ | — | — | — |
| CrunchBang Linux | — | — | GNU General Public License |
| CrunchBang Plus Plus | — | — | — |
| CRUX | — | — | GNU General Public License |
| CTRON | NTT, Inc. | — | — |
| Cub Linux | — | — | — |
| CubiOS | CubiOS Inc. | — | — |
| Cubuntu | — | — | — |
| Cucumber Linux | — | — | — |
| CutefishOS | — | — | — |
| CyanogenMod | CYNGN Inc. | 2009 | Android Open Source Project, Apache License, GNU General Public License |
| CyberOS | — | — | — |
| Càtix | — | — | GNU General Public License |