The skin produces natural oils to keep itself moisturized. People experiencing oily skin will produce sebum quicker. Those with oily skin, their face appears to have a sheen to it. Sebum is the substance your body makes, typically from the face and scalp, that helps keep those areas moisturized. For those with larger pores, the sebum production is even higher higher. Because of the extra sebum being produced, this can lead to frequent breakouts.
Issues that might arise are applying thick creams and moisturizers to the skin, causing breakouts. As well, over drying the skin of the oiliness can cause irritation, tightness of the skin, and dry flakes.