When a patch is applied, the inventory is changed and a snapshot of the inventory is stored it in $ORACLE_HOME/.patch_storage/<patch-id_timestamp>. This directory contains a restore.sh script which remove any changes that were made to the inventory after the application of the patch.
opatch util commands
Delete all inactive patches except the latest RETAIN_INACTIVE_PATCHES (as configured in $ORACLE_HOME/OPatch/config/opatch.properties) inactive patch with user confirmation:
opatch util deleteinactivepatches
Same thing, but without user confirmation
opatch util deleteinactivepatches -silent