DomainOS was an interesting, GUI-based alternative to Unix running on networked 68k-based systems (there was also a PRISM RISC-based DN10000 later) in the early 80s. Like so many other companies (TI's computer division, Convex, DEC via Compaq), hp took over and killed Apollo in the early '90s.
Another good source of information about Apollo systems and DomainOS is Jim Rees' site at https://jim.rees.org/apollo-archive/
You can run DomainOS emulated in MESS/MAME:
- https://www.youtube.com/watch?v=kdFEtuKaBdE
- http://mess.redump.net/howto/apollo