So far I know, OBERON was originallly written to run on an OS written in OBERON and has garbage collection.
It is however tightly coupled to this OS and needs a runtime system, if it runs on windows or other systems and does not produce stand alone programs or link to other languages. When executing it it is like using a virtual (Oberon) Machine, so far I remember it is decoupled from the host OS.
I might be wrong and dont know all flavors. I only tried Component Pascal (which is an OBERON flavor) on windows.
Installing it, I got a lot of anti-virus warnings, but I ignored it ;-) maybe this is owed to the runtime system.