Oily skin is the result of the oil (sebaceous) glands in the skin producing large amounts of oil.
Skin - oily
Your genes and hormones play a key role in whether or not you have oily skin. Diet probably has little, if anything, to do with whether or not you have oily skin.
The most important thing you can do for oily skin is to keep your skin clean using warm water and soap, or a soapless cleanser. Clean your face with astringent pads if frequent face washing causes irritation and increased oil production. Use only water-based or oil-free cosmetics if you have oily skin.
Your health care provider may also recommend or prescribe creams to absorb or limit the production of oil on the skin.
Oily skin may lead to acne.
Dinulos JGH. Acne, rosacea, and related disorders. In: Dinulos JGH, ed. Habif's Clinical Dermatology. 7th ed. Philadelphia, PA: Elsevier; 2021:chap 7.
Draelos ZD. Oily skin. In: Draelos ZD, Dover JS, Alam M, eds. Cosmeceuticals: Procedures in Cosmetic Dermatology. 3rd ed. Philadelphia, PA: Elsevier; 2016:chap 22.