I have extremely, extremely oily skin. I can blot my face one minute, and then 5 minutes later, blot and the tissue will be covered in oil...
So basically, in the morning, I would not recommend a moisturizer. A heavy one is definitely out of the question....What I do after I apply my acne gels and wash my face, is to use Neutrogena sheer sunscreen, because we need to wear sun protection everyday. This one is NOT oily/greasy, and I actually find it "drinks" up the oil and leaves my face dry BUT at the same time works as a type of sheer "moisturizer".
At night, you definitely should wear a moisturizer, especially if you're like me using so many heavy--duty skin cremes and topicals and oil-free face wash that can be drying to the skin. And at night, it doesn't really matter if your face gets a little bit more oily than usual because you'll be sleeping...I use Eucerin moisturizer...it's good, doesn't clog pores, kind of light weight..
For oily-skin girls, you need to make sure to moisturize once a day, because as someone mentioned, drying out the skin too bad, will cause your oil glands to react and over-produce oil, only going against what you're trying to achieve...so moisturize at night basically! We don't need to moisturize 2x a day like people with dry skin because our oils will come out and act as a type of moisturizer during the day..
Hope I helped!