The sebaceous glands, which secrete oil onto the skin, produce more oil than they should. The excess oil clogs pores. Heredity is a major cause, but diet and hormones affect it. Oil gland secretions can be increased by stress, hormonal activity changes, hot weather, pregnancy, or taking certain types of birth control pills.
Oily skin is most common among teenagers.
Keep your skin clean. Hot soapy water is even better. Ivory soap is a more drying soap than many others. Scrub the skin with soap and water. Some people put mud masks on to remove oil.