| Xandros | — | — | GNU General Public License |
| Xarnoppix | — | — | — |
| Xbox 360 system software | — | — | — |
| Xbox Linux | — | — | — |
| Xbox One and Xbox Series X/S system software | Microsoft | — | — |
| Xbox system software | Microsoft | — | — |
| XCP-ng | — | — | GNU General Public License, version 2.0 or later |
| Xebian | — | — | — |
| Xenix | Microsoft, Santa Cruz Operation, AT&T Corporation | — | proprietary license |
| Xenon Linux | — | — | — |
| Xenta OS | — | — | — |
| Xero Linux | — | — | — |
| Xerox Operating System | CII | — | — |
| Xevuan | — | — | — |
| Xiaomi HyperOS | Xiaomi | 2023 | — |
| Xiaomi.eu | — | — | — |
| Xinu | Douglas Comer | — | — |
| Xinuos OpenServer | Santa Cruz Operation, Xinuos | — | proprietary license |
| Xinux | UZINFOCOM | — | GNU General Public License |
| XMK | — | — | — |
| XOberon | ETH Zurich | — | — |
| XOS | — | — | — |
| Xous | Betrusted | — | Apache Software License 2.0 |
| XTOS | Tensilica | — | — |
| XTS-400 | BAE Systems | — | — |
| Xubuntu | Canonical Ltd., Ubuntu Foundation | 2006 | — |
| Xv6 | Massachusetts Institute of Technology | — | MIT License |
| ΜClinux | — | — | — |
| ΜnOS | — | — | — |