Excess oil in your hair usually occurs for two reasons: issues in the scalp or poor beauty habits. To get to the root of the problem, you’ll need to understand the science behind hair growth. Every pore on your skin has a sebaceous gland, including your scalp. The sebaceous glands secrete an oily substance called sebum, which gives your hair its natural shine. Sebum keeps your hair healthy and smooth, and it keeps the hair from becoming dry and breaking. However, some sebaceous glands produce an excess of oil, which results in oily skin and greasy hair.