After using the WebLogic Configuration Wizard to create an OAM Server cluster on two computers, and starting AdminServer, all servers start up properly. After shut down, a third server is added using the WebLogic Server Administration Console to create a new managed server and add it to the cluster. The third server goes into Running mode when started, with some exceptions in the start up log.
... Exception in thread "OAM Metric Persistence Timer"
in addition to the actions in the WebLogic Administration Console, you must register the server using the Oracle Access Management Console to ensure that the server can identify itself.
When adding and registering a second server instance for the same computer, all port numbers must differ: OAM Proxy port; the "port" that must match the one in the WebLogic Server Console; and the Coherence port.
For server registration details, see "Managing Individual OAM Server Registrations".