What is the role of the oracle in greek mythology?

Greek mythology tells of so many different oracles that there is no exact symbol to represent them all. The Delphic oracle, who originally belonged to Rhea, was taken by Apollo and is most closely associated with Python, the great snake that Apollo slew. Zeus's oracle resides at Dodona and is well known by its association with oak trees; it is said that the oracles of Dodona prophesy by listening to the rustling leaves of oak trees.