Take a look at Jeff Hunter's blog (Marist89), http://marist89.blogspot.com/2005/07/trace-it_04.html. He has a post about doing SQL tracing. Then start doing SQL tracing and see what your database is really doing. Also look at this page, http://www.psoug.org/reference/trace_tkprof.html. I would use this command to turn on tracing though, "ALTER SESSION SET EVENTS '10046 trace name context forever, level 12';"

Start by clearing out your $ORACLE_ADMIN/udump directory, turn on tracing during the busy part of the day, let it run for an hour, then turn it off and run tkprof on each of the trace files according to Jeff's post. Then look through each of the trace files.

Finally find the poorest performing SQL, tune, then start the process all over again. Continue until you have fixed the worst performing SQL. This might require adding indexes, changing SQL, changing table layouts and foreign keys or some mix of the above.