Oil on human skin is not a strange concept. Instead, it is an indication that such skin is healthy. The skin has pores. Each of these pores has a sebaceous gland right under it, which produces sebum. As a natural oil, sebum performs two major roles—maintaining the skin’s protective barrier and keeping it hydrated. But things can get overwhelming for the sebaceous glands. When this happens, they become overactive, thereby secreting sebum in excess. Excess sebum ultimately causes oily skin.