Oily hair has many causes. Genetics and hormonal changes, make some people to have more hair grease. Hair thickness and shape also affect hair grease. For example, fine-hair tends to be oily, while straight hair is often more oily.
Oily hair shampoos are usually milder, and can be used to cleanse your scalp and hair - gently scrub your scalp, as this helps to clean the scalp grease. You need to minimize using a hair conditioner - twice a week should be enough.
When using a hair conditioner, you should focus on the hair tips, not the scalp. Sometimes your hair is oily, but your scalp is dry. In this case, using tea-tree oil shampoos may be helpful.