Oily skin is hereditary and hormone dependent. It cannot be changed but can definitively be controlled.
Sebaceous glands are over active, when the oils come in contact with air they oxidize, they become blackheads. If the skin is either dehydrated or poorly exfoliated, the oils get stuck under the skin, bacteria invades, and you get a pimple.
The French secret is to avoid over washing, your skin always seeks balance, the more you remove oils the more it will produce!