Databases dont just consume CPU for no reason, they only consume because *something* is running.

Now that something is either:

a) you (ie, your applications, your users) actively using the database
b) us (ie, background tasks like calculation of statistics).

In either case, these will be running as database sessions.

So use whatever tools you have at your disposal to monitor which sessions are consuming resources. These tools could be:

- AWR reports
- v$active_session_history
- statspack
- home grown tools
- querying v$session, v$sql, v$sesstat
- auditing etc

Step 1 is to find out *what* and *who* is consuming resources. Step 2 is then to tune them.

But you need to do Step 1 first.