Analyze your tables and indices. When you analyze, choose the estimate statistics, instead of compute statistics. Also set the CURSOR_SHARING from exact to force. IF your optimizer is choose, I would suggest you to change it to FIRST_ROW. But you would want to leave it as CHOOSE if it were of DWH database.

You can also check the type of queries that had been run on your database from querying the V$SQLAREA. Then decide what to do from that point.

Good luck,
Sam