Oily/greasy hair can be a product of many factors. Natural causes include hair type, genetics, hormones may play a role in how oily or greasy their hair is. If they experience severe cases of oily and greasy hair, it is best to advise them to seek a medical professional for treatment. Other factors that contribute to oily and greasy hair can be preventable. These include brushing hair too frequently, shampooing and conditioning hair too frequently, using too much product, or tying hair up too much.