An oily skin is caused by the secretion of too much sweat keratin sebum melanin

Oily skin is caused by the secretion of too much sebum which is the main substance produced by sebaceous glands of the skin. Sebum is mostly composed of fat and other non-polar compounds which makes it "oily". Sebum protects the skin from losing moisture from evaporating into the environment as well as keeping the nourishment of the skin in place.