Oolong is a semi-oxidised tea ranging in oxidation from 10%-80%. This is why some oolongs are quite green, whereas others are quite dark, giving oolongs much variance in flavour profiles.
In China, tea has historically been used as medicine. Oolong derives mainly from the Fujian province in China, as well as Taiwan. Oolong is commonly drank for it's delicious unique flavour as well as it's many health benefits.
Oolong contains high amounts of polyphenols which enhance the function of enzymes and helps to remove free-radicals. It has also been associated with weight loss because it contributes to the function of fat metabolism to aid in controlling obesity.