Oolong teas are semi-oxidised teas, they are neither green nor black but a category of its own. Oxidation is a process that exposes tea leaves to air which in turn contributes to its dark colour and aroma. Similar to how an apple turns brown when its left out in the air for a period of time. Green teas are barely oxidised while black teas are fully so. Lighter Oolongs are oxidised for a shorter period, while darker Oolong teas are oxidised longer.
Tea leaves undergo oxidation and fermentation to bring out their unique aromas and flavours. What is the difference between oxidation and fermentation in teas?