The difference between green tea and oolong tea
To make green tea, the tea leaves are harvested and dried in an iron pot over a fire.
The Chinese noticed that there were complex oils in tea that were sensitive to air, so they tried allowing the leaves to wilt by using a much slower drying process. They bruise and soften the leaves to allow the oil to oxidize. That's how oolong (black dragon in Chinese) tea is made.