Project Oberon
The Design of an Operating System,
a Compiler, and a Computer
Revised Edition 2013
Niklaus Wirth
Jürg Gutknecht
ISBN 0-201-54428-8
http://www.inf.ethz.ch/personal/wirth/ProjectOberon/index.html
http://projectoberon.com/

Emulators for the Oberon RISC machine:
https://github.com/pdewacht/oberon-risc-emu
http://schierlm.github.io/OberonEmulator
https://github.com/MGreim/riscpas_repo
https://github.com/aixp/ProjectOberon-BlackBox
http://www.informatik.uni-bremen.de/~fld/UnixAos/RISC-Oberon/

Related:
https://github.com/andreaspirklbauer/Oberon-experimental