I found the Project Oberon book in SFSU library and read it cover to cover. Such a tight, elegant system. Pretty much complete: Oberon OS includes compiler for Oberon language!
Language was self-hosted (compiler written in itself) and Prof. Wirth had a heuristic: no feature could be added to the lang/compiler if it made compiling the compiler slower.
There are several emulators, in C, JS, Java, and Python, and there's Verilog so you can burn to FPGA and make a real workstation. People have.
The system has two kinds of OOP. There is the C++/Java/Python/etc. kind where you basically add methods to a record type, and there is the old school Smalltalk message-passing kind, where elements of the OS communicate through an extendable message bus.
The GUI system, especially with the "Gadgets" subsystem, is mind-blowing. It would take too long to go over it at the moment, but there are features there that still haven't made it into mainstream GUI systems. If I get a minute later today I'll come back and add a note detailing some of the awesome if someone else hasn't already filled in the picture.