Oolong tea is a product made from the leaves, buds, and stems of the Camellia sinensis plant. Oolong tea is used to improve mental alertness. It is also used for obesity and heart disease, to prevent weak and brittle bones (osteoporosis), and for other conditions, but there is no good scientific evidence to support these other uses. Mental alertness. Drinking oolong tea or other caffeinated beverages throughout the day seems to help maintain alertness and mental performance. Combining caffeine with sugar as an "energy drink" seems to improve mental performance better than either caffeine or sugar alone.