Oily skin doesn't necessarily cause acne, but it can be a contributor to breakouts if the skin isn't kept clean enough. Acne occurs when pores are invaded by dirt and oil, get clogged and breed bacteria. The bacteria in the pore causes swelling and the typical pimple is filled with pus, which is the body's attempt to fight infection using white blood cells. Keeping your skin clean and pores unclogged is one important way to prevent acne breakouts. And it all starts with the way you wash your face.
http://www.lifemojo.com/lifestyle/tips-to-take-care-of-oily-skin-103174704