The causes of an oily forehead include heredity and endocrine, bad habits, poor diet and other factors. 1. Heredity and endocrine: If you have oily skin, your sebaceous glands will produce a lot of oil on your face, leading to an oily forehead. 2. bad habits: long stay up late, irregular life and other factors may lead to excessive oil secretion, thus triggering the forehead oil. 3. Poor diet: If you eat less vegetables and fruits, and enjoy foods such as chili peppers, fatty meats, and chocolate cake, your skin may become oily more easily. There are many reasons for an oily forehead. It is recommended that you eat more fresh fruits and vegetables, have a regular routine and keep your face clean to help improve the symptoms of oiliness.