Emily, have you tried trisodium phosphate (TSP). You can get it in the paint department of any major paint retailer.
Spray a little water on the area then sprinkle the TSP out of the box onto the oil stains. Let this set for 15 minutes (less in hot sun--you don't want it to dry). Then scrub with a long handled, stiff brush. Don't try to do this with a small brush while kneeling. The concrete and TSP will tear up the skin on your knees. Finally, rinse well with the hose. If you have fresh oil that is still liquid, get a bag of the cheapest kitty litter you can find and sprinkle that on an hour before you start the TSP treatment and sweep it up right before you start the washing.