By default, the Oracle Exadata (oracle.sysman.xa) and Oracle Database (oracle.sysman.db) plug-ins are deployed to your OMS during the OMS installation, and the parts of the plug-ins necessary for discovering the targets of the types are deployed to an agent when you install/deploy the agent. Thus, normally it is not necessary to manually deploy the plug-ins to an agent.
You may want to make sure that your OMS has the latest versions of the Oracle Exadata and Oracle Database plug-ins before discovering the targets. As of now, the latest version for both plug-ins is 126.96.36.199.
Note: The current version of Oracle Exadata Healthchecks plug-in, which is optional, is 188.8.131.52.