What prerequisites package is needed for Oracle 11g + 12c on the same OL7 server?
The problem with oracle-database-server-12cR2-preinstall is that it does not install gcc. For a base release, it is fine, but upon installing a PSU/RU, opatch fails because of that.

[root@o61 ~]# yum deplist oracle-rdbms-server-11gR2-preinstall-1.0-14.el6.x86_64 | grep dependency | grep gcc
  dependency: gcc-c++
  dependency: gcc
  dependency: libgcc
[root@o61 ~]#

[root@o71 ~]# yum deplist oracle-database-server-12cR2-preinstall.x86_64 | grep dependency | grep gcc
  dependency: libgcc
[root@o71 ~]#


It is not needed to install both packages. I would just do this (as I usually do):

yum install oracle-database-server-12cR2-preinstall gcc gcc-c++ -y