Centos 5 is very old. If you want to install the server, choose Centos 7 that is supported by RedHat and by OCS Inventory. We provide also RPM to made an easy install.
Centos 7 is a really good distribution with a long support by the editor. Think to upgrade your server.
If you want to install the agent, you must use the tar.gz manual installation. We don't provide RPM as centos 5 is not supported anymore by RedHat. The agent must run whitout problem on Centos 5.