Pretty long awaited second release of 12c (On-Premise) version is (apparently) officially available for download at: http://www.oracle.com/technetwork/database/enterprise-edition/downloads/index.html

Currently Oracle Solaris SPARC, Oracle Solaris x86-64 and (of course) Linux x86-64 versions are available.
As always I suggest to read a New features Guide in order to explore new things brought by current release: http://docs.oracle.com/database/122/NEWFT/toc.htm

Enjoy learning and exploring...