User systems (onboard laptops etc.) on the Space Station runs Linux. Used to be WinXP. SOURCE

The Space station itself probably runs on VERTEX/VxWorks as Curiosity and a few other space projects do.
VERTEX/VxWorks Wikipedia

Curiosity Link

VxWorks Link

In the early 90s I had my hands on VERTEX for a while. I was told to study it. When we wanted to start a project and called the VRTX helpdesk with our serial number and all we were told that we have an illegal copy, the real owner was some military type company in the UK. That was the end of it. The software came on floppy disks and was the entire operating system in Assembler code with manuals on how to use it. They company paid R 50 000 for it in the 90s !. Pity I didn't make copies.