One of the Plan 9 papers actually refers to Oberon as an example of a similarly integrated system. I believe it's the one titled 'The Use of Name Spaces in Plan 9,' but I'm not certain.

Unfortunately, the server is down right now so I'm unable to check but, if you're curious, the various papers can be found here: http://plan9.bell-labs.com/sys/doc/

For over a decade, dozens of actual people used Plan 9 as their main environment. (Many but not all of these people worked at Bell Labs.) I tend to believe that Smalltalk has also seen serious numbers of actual users. In contrast, who has ever used Oberon as their main software environment except perhaps students required to do so to pass some class?