Using EM12c corrective action of type SQL Script you can automate the cleanup of the database OS audit file that are created in the adump directory.
If you want details on how to create and deploy corrective action use my blog post here
Below is the script that can be used in the SQL Script corrective action to delete anything older then sysdate-1 in the adump directory
WHENEVER SQLERROR EXIT FAILURE;
audit_trail_type => DBMS_AUDIT_MGMT.AUDIT_TRAIL_OS,
last_archive_time => SYSDATE-1);
audit_trail_type => DBMS_AUDIT_MGMT.AUDIT_TRAIL_OS);
The metric that monitors the adump size of audit file is below so you can add the corrective action to the metric.