At the raw metal level, zOS. But [you can run Linux on top of that](https://www.ibm.com/it-infrastructure/z/os/linux https://www.ibm.com/it-infrastructure/z/os/

At the raw metal level, zOS. But [you can run Linux on top of that](https://www.ibm.com/it-infrastructure/z/os/linux https://www.ibm.com/it-infrastructure/z/os/linux].