I'd like to suggest CentOS 6.5 if you're having trouble with OEL...
Your error says it all-the installer cannot read /mnt/runtime/var.
Are you installing in virtualbox or vmware? I've never had this issue installing Red Hat, or CentOS on vbox or vmware or KVM. I've never installed OEL, though. I assume it uses roughly the same installer/UI that RHEL and CentOS do. How far along is this error message?