ORACLE Audit logs
Oracle by default only audits very few events unless configured. http://docs.oracle.com/cd/B19306_01/network.102/b14266/cfgaudit.htm#i1007559
enVision-Oracle guide has some examples (p6-p7) but before configuring I suggest you to go through link:
You can also read up about enabling privileged auditing here:
Access based auditing fills up database pretty quickly so we often suggest DBAs to configure auditing based on their environment.