If you want to install Oracle 12c on EL6 Linux Systems there are many pre-install steps that you should do before actually begin the database installation.

For ease Oracle provide as name states RPM package: oracle-rdbms-server-12cR1-preinstall
But it is only for Oracle Linux from Oracle Public YUM Repo.
Ofcourse you can activate this repo without trouble but in fact problem of this RPM is dependency of Oracle Linux Kernel exactly UEK. Output:

Installing:
 oracle-rdbms-server-12cR1-preinstall    x86_64    1.0-8.el6                  /oracle-rdbms-server-12cR1-preinstall-1.0-8.el6.x86_64     37 k
Installing for dependencies:
 compat-libcap1                          x86_64    1.10-1                     base                                                       17 k
 compat-libstdc++-33                     x86_64    3.2.3-69.el6               base                                                      183 k
 kernel-uek …