Oily hair is really an oily scalp. Wash your scalp and hair as needed, lathering twice if you need to. Try leaving the shampoo on your head for at least five minutes before rinsing. And condition your hair on the ends only. Do not place conditioner directly to your scalp. Avoid brushing your hair too often, as the brush brings oil from the scalp to the ends of your hair.
An oily-hair "quick-fix" is to dab a bit of talcum powder to the roots of your hair.