More so than for Black or Green Teas, the soil and the climate play a big part in determining the final product. So, look for the best of Oolong Tea coming from Taiwan, formerly Formosa .
Oolong teas go through several steps in their process.
After plucking they are withered in the sun for up to an hour and then further withered inside at room temperature. As this takes some time, the natural fermentation gets started turning the leaves red and developing the characteristic fragrance, almost peachlike, before it is stopped in pan firing. After the first firing, the leaves are rolled, fired a second time. Once again they are rolled and fired for a final time. This partial fermentation results in the development of some essential oil but not in the high concentration seen in Black Teas where full fermentation occurs.