| 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 |
| Cray Operating System | Cray | — | — |
| Cray Time Sharing System | Lawrence Livermore National Laboratory, Los Alamos National Laboratory | — | — |
| CTRON | NTT, Inc. | — | — |
| CubiOS | CubiOS Inc. | — | — |
| Càtix | — | — | GNU General Public License |
| Dartmouth Time Sharing System | Dartmouth College | — | — |
| Data General AOS | Data General | — | — |
| DC/OSx | Pyramid Technology | — | — |
| DEC BATCH-11/DOS-11 | Digital Equipment Corporation | — | — |
| DEFT Linux | Stefano Fratepietro | — | GNU General Public License |
| DEMOS | National Research Centre Kurchatov Institute | — | BSD licenses |
| DerrickOS | Jozef Bogin | — | — |
| DesktopBSD | — | — | BSD licenses |
| DexOS | — | — | — |
| DG/UX | Data General | — | — |
| DigitaOS | — | 1998 | — |
| DioneOS | — | — | — |
| DIP-DOS | — | — | — |
| DiskStation Manager | Synology Inc. | — | — |
| DNIX | Dataindustrier AB | — | — |
| Dolphin OS | — | — | — |
| Domain/OS | Apollo Computer | 1981 | — |
| DOSB10 | — | — | — |
| DR-DOS | DRDOS Inc, Digital Research | — | — |
| Dreamcast system software | Sega | — | — |
| DreamShell | — | — | BSD licenses |
| DSOS | Texas Instruments | — | — |
| DuinOS | — | — | GNU General Public License |
| Dusk OS | — | — | GNU General Public License, version 3.0 |
| DX-DOS | — | — | — |
| DX10 | Texas Instruments | — | — |
| Dynix | Sequent Computer Systems, IBM | — | — |
| ECos | — | — | eCos-2.0 |
| Edos | The Computer Software Company | — | — |
| EdulibreOs | — | — | — |
| Edutice | — | — | — |
| Elastix | — | — | GNU General Public License, proprietary license |
| Elbrus Linux | Moscow Center of SPARC Technologies | — | — |