Z/OS is IBM's mainframe operating system.
This manages most of the infrastructure of our world.
Just to give you an idea how inportant they are.
They have been around since the mid 50's if you count R&D
Bill O'farrell and his team talk about how they changed Z/Os forever.
Want to hack on one?
Check it out for 120 days:
https://developer.ibm.com/linuxone/