Here is a good discussion of the question, with some interesting suggestions for taking care of the oil stain:
One product I've heard mentioned a bunch of times that works really well is called Bulldogs Grease Away. Haven't used it myself though.
Wouldn't ever recommend anything called "Bulldog"...Go Gators!
'Degreeser' is available at The Depot or paint store. Pour it on, scrub it in repeatedly. If you think you have it all, wash it off with soap and water. It can go way down into the concrete, so you may need to repeat this but the concrete needs to be dry again before the degreeser can work.