So in order to start the database install without trouble, these few simple steps will get you going :
If you want to create a virtual machine
You can do a minimal installation of OL6, or any other installation that you prefer (Desktop, Workstation, etc). The install by default, if you don't register with ULN, will point to our free public-yum repository with all the latest RPMs (errata, bugfixes,...). It might make sense to run yum update although you don't really have to.
Then just install the oracle-rdbms-server-preinstall RPM and your OS is completely configured to start the Oracle database installer. Simply type yum install oracle-rdbms-server-12cR1-preinstall and it will download all required dependencies, create the oracle user id, modify sysctl.conf and modify limits.conf.
Next, download the 12c R1 software, start the installer and you're good to go.
We make it easy, or at least try to :). enjoy.