I noticed that OEM 12c console was very slow a few hours after the upgrade to 18.104.22.168 version.
Looking at the repository DB, found several OMS sessions consuming significant CPU resources.
Bug 19199023 explains that some SQL queries executed against the repository consume high CPU on the servers. This bug affects the DB plug-in 22.214.171.124 and the patch 19176910 should be applied to the plug-ins.
More information available on MOS note, 126.96.36.199 OEM: High CPU utilization on Repository Database due to SYSMAN query WITH TARGETGUID AS (SELECT target_guid, host_name FROM mgmt_targets (Doc ID 1912172.1)