| 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 |