Each server must have an operating system that is certified with the Oracle Clusterware version you are installing. Refer to the certification matrices available in the Oracle Grid Infrastructure Installation Guide for your platform or on My Oracle Support (formerly OracleMetaLink) for details, which are available from the following URL:
http://www.oracle.com/technetwork/database/clustering/tech-generic-unix-new-166583.html
When the operating system is installed and working, you can then install Oracle Clusterware to create the cluster. Oracle Clusterware is installed independently of Oracle Database. After you install Oracle Clusterware, you can then install Oracle Database or Oracle RAC on any of the nodes in the cluster.