Concrete garage floors and driveways absorb any type of liquid spilled or dropped onto their surface, including oil. Fresh oil spills are easier to clean than older spills because you have time to absorb the oil, but it's possible to clean regardless of how long the oil is there.
Oven cleaner works well to remove older oil stains, as it reaches past the top layer of the concrete. For fresh oil spills, use a dish soap designed to cut through grease and tap water.
For oven cleaner, apply a thin layer of the cleaner to the top of the stain, cover with a tarp and wait 24 hours. For newer stains, apply dish soap and pour a small amount of water on top. Gently work the soap into a lather and rinse with fresh water.
If the oil stain is fresh, apply cat litter or sawdust over it, which absorbs the oil. Sweep the sawdust away and then clean. For older or dried stains, clean as directed and then use cat litter or sawdust to absorb the liquid.