Oolong Teas generally come from China and Taiwan (formerly Formosa) and, as a category, they fall between black and green teas. They are partially oxidized in the range of 5% to 95% and may undergo varying processing methods, both of which result in varying complexities in aroma and flavor. The level of caffeine ranges between Black and Green Teas and is determined by the amount of oxidation the leaves have undergone.