A cup of brewed oolong tea contains small amounts of calcium, magnesium, and potassium. It also contains about 38 mg of caffeine. In comparison, a cup of green tea contains about 29mg of caffeine. Some of the main antioxidants in oolong tea, known as tea polyphenols, are theaflavins, thearubigins, and EGCG. These are responsible for many of its health benefits. EGCG is one of the most powerful compounds in green tea. Research has tested its ability to help treat various diseases. It appears to be one of the main compounds give green tea its medicinal properties.