I have been tasked with identifying ways we can tune our Oracle database for better performance. First, I want to know what are the most common causes of oracle db performance problems? What should I be looking for and when?
It's impossible to answer this question in a few sentences. Your best bet is to consult one of the excellent performance tuning books currently on the market. I particulary recommend Cary Millsap and Jeff Holt's Optimizing Oracle performance, and Oracle performance tuning 101, by Vaidyanatha, Deshpande and Kostelac.
Briefly, though, the most common causes of Oracle performance problems are applications that cause the database engine to do much more work than is necessary to achieve the desired result; poorly-designed database schemas; and poorly-designed databases. These can manifest themselves in many ways: excessive CPU consumption due to too many logical I/Os (visits to database blocks in memory), excessive disk reads due to missing indexes, or excessive contention for shared resources.