Starting with 11g, Oracle is keeping the logs in check with a default deletion policy of 30 days for dumps and one year for the rest. The mmon process is deleting them.
Log rotation for Oracle 11gR2 database
https://silviumc.wordpress.com/2013/06/05/log-rotation-for-oracle-11gr2-database/
rotate all the log files in the RAC environment
http://www.dbaexpert.com/blog/rotate-all-the-log-files-in-the-rac-environment/
