The article is the best introduction and overview I know. There are many more comprehensive sites.

The revival is worth a look:

http://www.projectoberon.com/

This academic paper is highly readable and interesting:

https://news.ycombinator.com/item?id=15704806

Wikibooks has excellent docs in depth:

https://en.wikibooks.org/wiki/Oberon

But I often cite the Ignore the Code post when someone doesn't know what Oberon is. I used it in a comment elsewhere on HN, and thought "why not submit it?" and here we are. The word "language" wouldn't fit in the title, AFAICR.