|don't let this picture fool you, my forehead could join OPEC|
Start with your skin by applying an oil reducing primer or lotion. I'm fond of Clean and Clear Finishes Mattifying Moisturizer because it is not overly drying. After applying my makeup I finish it off with a mattifying powder like Fyrinnae Fluff, Mattify Cosmetics Mattify Ultra, Maybelline Dream Matte, etc. This will help keep the oil at bay for as long as possible.
Next, it's prepping the hair. If your hair is already greasy it's best to wash it, if not move along. My "secret weapon" of sorts is Fyrinnae Fluff, it's a silica powder for your face but I like to powder my bangs with it. It keeps them looking fresh ALL DAY with out the white cast that dry shampoo gives. Even when my forehead has become an oil slick Fluff helps protect my bangs from absorbing it.
Speaking of which, it is inevitable that your face will regain its oily shine eventually even after all this prep work. Blotting papers are one of the best solutions as they actually remove the oil from the skin. They are rather cheap and pretty easy to come by as well. After blotting away as much oil as possible I re-powder with whatever mattifying powder I have available (usually the now discontinued but amazing Hard Candy one since it lives in my purse).
While conventional wisdom says you can't have bangs with an oily face I say go for it if you really want them.