The main cause of oily hair is when the sebaceous glands in the scalp produce excess sebum. Glamcheck says that the hair absorbs the excess sebum and produces greasy hair.
Oily hair can also be caused by triggers. If the skin of the person is oily, they are more likely to have oily hair. Another factor to examine is dry scalp. A dry scalp triggers the sebaceous glands to produce more sebum to give moisture to the scalp. Do not shampoo the hair every day to treat oily hair. According to Glamcheck, this can cause dry scalp and make the condition worse. Switch to a mild or clarifying shampoo, and use it every other day.