After you upgrade Oracle Database, complete required post-upgrade tasks, and consider recommendations for the new release.
Patch Set Updates and Requirements for Upgrading Oracle Database
Before starting upgrades, update your new release Oracle Database to the latest Oracle bundle patch, patch set update (BP or PSU), or Release Update (Update), or Release Update Revision (Revision).
The software for new Oracle Database releases contains a full release that includes all the latest patches and updates for Oracle Database at the time of the release.
Before you start an upgrade or downgrade process, Oracle strongly recommends that you update both your earlier release and your new release Oracle Database. For Oracle Database 12c or earlier releases, update to the latest Oracle bundle patch, or patch set update (BP or PSU). For Oracle Database 12c release 2 (12.2), Oracle Database 18c, or later releases, update to the latest quarterly Release Update (Update) or Release Update Revision (Revision).
My Oracle Support note 854428.1 contains information about patch sets and updates.
My Oracle Support note 730365.1 contains an upgrade reference list for most available Oracle Database releases, including download information, patch numbers, and links to other notes.
My Oracle Support note 2180188.1 contains lists of one-off patches for upgrades, downgrades, and coexistence with previous releases.
My Oracle Support note 1227443.1 contains a list of Oracle Database PSU/BP/Update/Revision known issues. This note provides information about all known issues notes for Oracle Database, Oracle Grid Infrastructure, and the Oracle JavaVM Component (OJVM).
My Oracle Support note 2118136.2 contains a download assistant to help you select the updates, revisions, Patch Set Updates (PSU), SPU (CPU), Bundle Patches, Patchsets, and Base Releases that you need for your environment. Oracle highly recommends that you start here.