Oracle DBMS - Secure The Infrastructure Housing Your Database
Make sure the Oracle host is in a locked room with limited access, redundant power, and fire protection systems. Don't just assume that this is the case. Check it out yourself or confirm it with the administrator responsible for that machine.
Also take a look at the hardware strategy involving the database. Security isn't only about hackers; it's also about availability of the database. If you aren't using RAID or some other disk-mirroring solution, you should do so. Look at disaster recovery and determine what would happen if the Oracle host crashed and was offline for several days.