Oily skin can be a problem for many reasons but one major is that excess oil makes more breakouts. Combination skin tends to be oily only in the T-zone. This is because there is more concentration of oil glands on the forehead than on the cheeks. Water glands, on the other hand are all over our body but if they are a little underproductive on the cheeks then the water will evaporate quicker than it would on the forehead, leaving dry skin. You won’t dry out in your t-zone though because there are so many oil glands that the water gets protected by oil sitting on top.
There are a couple ways to treat this type of skin. One way is to use two different types of cleansers to wash. Use an acne wash on your t-zone and a mild, lotion soap on your cheeks. If you are not extremely dry on your cheeks then you could use the acne wash all over your face and then quickly replace moisture to cheeks with a moisturizer.
And of course, if you have oily skin all over your face, you should use an acne cleanser to wash with and a moisturizer that is water based and oil-free. Oily skin can really benefit from a clay face mask too. The clay will draw out impurities and leave skin feeling very refreshed.