Oily skin is caused by the overproduction of sebum. The skin looks shiny, make up tends to slide off the face, you may notice visible pores and may experience spots. Oily skin can't turnover as effectively, causing dullness and a build up of excess skin cells.
Avoid stripping the skin with harsh products. Instead, focus gentle, but effective cleansing (morning & night - the goal is NOT "squeaky clean" skin!), lightweight hydration, and gentle exfoliation (2-3 times a week).