The Oberon System (documented in the Book "Project Oberon" by Niklaus Wirth and Jürg Gutknecht) originated in 1985 (see also http://en.wikipedia.org/wiki/Oberon_operating_system). End of the 1980ies and in the 1990ies it was ported to several other operating systems as an application running on top of these OSs.
The sources where not distributed. Yet they were passed sometimes on. In the late 90s ETH put Oberon under a permissive license and a private web site started hosting them. ETH itself - probably lacking interest in the early version of Oberon - never updated their FTP server.
This sourceforge project contains now one download and two repositories. The download section and the CVS repository Code were originally created by Robert Lichtenberger for his version of Oberon V4 for Linux. They still contain Robert Lichtenberger's Oberon V4 for Linux.
In 2013, Claudio Nieder made his collection of different sources of Oberon V4 available in the
repository "V4 Sources". It contains sources for different platforms. This is a git repository which is best checked out as it contains many symbolic links.
Some of the Modules of which the Oberon OS is built are common to several Oberon V4 implementations. These are found in the directory named Common.
Whenever another implementation's version of the module is such a "shared" one, the specific directory (e.g. Linux) holds just a symbolic link to the Module source in Common.
Some more information on the Oberon System and its history can be found on Wikipedia.