Ezr 2:63 And the Tirshatha said unto them, that they should not eat of the most holy things, till there stood up a priest with Urim and with Thummim.
Ne 7:65 And the Tirshatha said unto them, that they should not eat of the most holy things, till there stood up a priest with Urim and Thummim.
This breastplate had the 12 jewels representing the 12 tribes. It was a ceremonial garb and the priest wore it on special occasions. To this day, I think we have to admit that it is still unclear to us, all that it represented. However, as "a divine oracle" we have to understand that only one Spirit, God's Spirit was involved, as opposed to other oracles of the heathen who would have used their own imaginings in divination, or perhaps, did consult a spirit, but certainly not God's Spirit.
For newcomer lurkers: Jewish Encyclopedia entry on the subject