First off this isn't really the correct forum - for this question, you would have been answered much more quickly in the Database General forum.
You have a few choices for this:
1. Enable auditing (guessing you don't already have this), have a look at http://docs.oracle.com/cd/E11882_01/network.112/e16543/auditing.htm#DBSEG376
2. Setup a DDL trigger to log auditing in a customized way, have a look at http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/triggers.htm#CIHEFBJA