Most OS we use today are UNIX/Linux based or Windows. Both originates from USA. In fact, most of them are UNIX derivatives, so Bell Labs have a powerful influence on the design of the OS as most OS I see are influenced by UNIX in some way. What were the other R&D companies and universities around the world doing at that time? I think some of them might have developed an OS of their own, it just didn't became popular or some other reason...
j
stuff ;)
Project Oberon by Niklaus Wirth one of the few real fullstack developers ever. He built the chips, wrote the operation system and the programming language ontop of it.
There is Riot OS I know of, which is for embedded devices (there are more)
There is redox redox-os.org for rust
There are many more :D but those are the ones that actually came to mind.