Two ideas. Both work. My first choice would be to throw down cat litter over the oil spot. The litter is an absorbant. If you do this, let it sit 3-4 hours, sweep it up, and apply more litter, but this time grind it into the oil spots with your foot. this will get the litter material into the pores of the concrete and wick out the remaining oil. Might have perform the second step another time, This works best on fresh oil spots.
The second choice is to use a paste of laundry detergent and water, then scrub like mad. Let it ist for 1/2 hour, add a bit of water, scrub again and rinse clean. Not the most environmentally friendly solution, but it works.