I had access to the Native Oberon version during the 90's.

Oberon was inspired by Mesa/Cedar, there are quite a few Xerox papers about how Mesa/Cedar used to work. This is probably the most relevant one.

Think about a Lisp/Smalltalk environment, but based on a strong typed language instead.

Regarding Oberon, it had similar ideas.

Basically there were no programs, only modules (strong typed dynamic libraries).

Any procedure/function that had a special type signature could be called from the CLI environment, UI action, or after selecting an UI element, depending on the way the mouse buttons were used.

Also the environment was focused on graphics not plain text.

The last version before Active Oberon was introduced, System 3 with Gadgets, had quite nice Amiga style GUI.

You can find tons of screenshots on my site.