The command yum install oracle-rdbms-server-12cR1-preinstall is all you need to prepare your system for Oracle 12c installation. It will trigger the installation of the prerequisite packages, and configure the Oracle account and kernel parameters.
Regarding the installation source, see the Oracle Linux 6.4 release notes, chapter 3.2.2 and 3.2.3. Yum is the package managment tool which retrieves packages from the Internet using html or the local DVD media. The oracle-rdbms-server-12cR1-preinstall package was released after the 6.4 media, but you can get it via yum configured to connect to Oracle Public Yum (default).
And btw, you might want to use the right nomenclature. The product is called Oracle Linux and not Oracle Enterprise Linux anymore, which changed during the release of Oracle Linux 5.5.
Thanks so much. Yes, I found my internet connection is up even though I do not have firefox or any browser installed. I was able to ping google.com and yahoo.com so I can use yum. Also, thanks for pointing name change to Oracle Linux.