One of the very best guys in Oracle internals is Tanel Poder. He taught a class 2 odd years ago ( Hotsos sponsored ) by the DC airport that was extremely good.
One of the things Tanel does is offer a nice set of sql command line scripts that help one diagnose performance anomalies and database strange events. His snapper script just by itself is well worth checking out.
He has been blogging more about various problem scenarios and how to investigate them and traveling and teaching a little less recently. It probably gets old flying too much.
He is offering classes remotely now and has the first week of his Oracle Advanced Troubleshooting coming up ( tomorrow ... monday april 11th 2010 ). You can find more information here Tanel Poder training seminar ...
He has two main sites e2sn and his blog tanels blog ... and he is definitely worth taking your time to check out.