Clicking on Oracle RAC OS metric link shows "This Host is Not Currently Being Monitored". OS metrics on the database dashboard is working fine. The related host metrics are displaying fine on the Hosts dashboard.
The Oracle RAC node agents were created using the IP addresses while the Hosts Infrastructure agents were created using the hostnames.
Using IP addresses to create the DB agents and hostnames to create the Infrastructure Hosts agents is not recommended. Either use all IP addresses or all hostnames. The workaround is as follows but it will remove the historical data of the Infrastructure Hosts model for the related hosts.