Natural oils accumulate on the scalp and spread down to the hair shaft over time, so washing your hair every day will make it less oily. It is, however, possible to over-wash your hair and strip all the important natural oils, which serve to preserve luster and general health of the hair. Once a day, or once every other day depending upon your hair type, is certainly reasonable to protect natural oils and prevent any greasiness of your scalp.
If this still results in a greasy scalp, you may need to talk with a board-certified dermatologist about topical preparation to help specifically decrease the amount of sebum production from glands on your scalp.
