Got a number and want to put it in a string - Oracle has to do a conversion. Not a big deal but there's a small and easy to remember detail which leads to slightly faster code. Look at this article to see how the performance can improve by avoiding unnecessary data type conversions.
Get information about waits and other events of your database by using ASH (Active Session History)
Finding a performance issue can be a tricky thing but if you know the tools available at Oracle, you'll sometimes even enjoy it. This article shows you how you can analyze a PL/SQL procedure or function.
Oracle allows you to compress indexes for a rather long time, but a lot of DBAs still don't know about that feature yet and they probably should.
You'll definitely have to use indexes if you want your database to perform but having too many indexes isn't good either. Save some storage and make your database faster by removing unnecessary indexes.
An old feature but rarely used - the native Oracle compiler which will simply make things faster with almost no effort