Oracle has released the new 12cR1 version of their database for Linux on System z. With the appropriate license you can download it from Oracle. There is a also a blog entry in the Oracle blog. For more details see this IBM Flash.
The Redbook "Experiences with Oracle Database 12c Release 1 on Linux on System z" is a "must read" if you want to start with Oracle 12c on System z.
In the IBM Germany Lab in Boeblingen there has been a class called "Oracle Bootcamp". The presentations can be downloaded here.
Also for the older version 11R2 Oracle released the 11.2.0.4 patchset. There is also a new Redbook available for Oracle 11: Experiences with Oracle 11gR2 on Linux on System z.
(updated 3/26/2014)