Go to your local hardware store and get a bottle of muratic acid. Wet the concrete first. Mix the acid 50/50 with water and brush on the oil spot. Watch it bubble a few minutes and rinse completely with water. Note: Muratic acid is nasty stuff. Rubber gloves & eye protection along with long sleeves & slacks are a must. You can actually etch concrete with this acid so if you are using it on a steel troweled garage floor then mix 1/3 acid to 2/3 water. Do not use in direct sunlight and do not let the acid dry on the concrete.