Like Green Tea and Black Tea, Oolong Tea can be processed in a variety of ways. But what makes Oolong Tea Oolong is the leaves are left out in the air for a period of time. This allows the leaves to ferment some. Whereas Black Tea is left out for approximately 48 hours or more, Oolong is typically left our 16 to 24 hours. However, Oolongs can range anywhere in between Green Tea and Black Tea in terms of fermentation with some Oolongs on the Black Tea side of the scale and some Oolongs on the Green Tea side of the scale.