Could changing the cluster name for a OAS instance in opmn.xml create problems for existing appli (4262954)

The Foglight - Managing Java EE Systems - Installing the Java EE Management Capabilities Guide says that the cluster name of an OAS instance (OC4J) needs to be changed from its initial setting of default to identify that instance as part of a cluster. Will changing this have any effect on application deployed on this instance

Not normally. The only way this might if the application in same way depended on having the cluster name set to default. However this is only something the application developers could answer for sure.