The initial Oberon OS is indeed from 1987, but there are many Oberons. The original Oberon GUI was the everything-is-executable-text one, not the ZUI.

The ZUI was part of Thomas M. Frey's work for his dissertation "Bluebottle : A Thread-safe Multimedia and GUI Framework for Active Oberon" [2] (Bluebottle OS) submitted in 2005.

This was built on top of Pieter J. Muller's work for his dissertation "The Active Object System" [1] (AOS) submitted in 2002.

There are many more ETH (PhD and student) projects based on Oberon before and after (ARM Oberon, WinOberon, UnixOberon, Oberon.NET, ...) mainly in Jürg Gutknecht's group.

[1] https://www.research-collection.ethz.ch/handle/20.500.11850/... [2] https://www.research-collection.ethz.ch/handle/20.500.11850/...