I'm not a big fan of power washers, but that would be easier. A mop isn't going to give you enough friction to remove the dirt - a long handled brush would be better if you go that route.

Whichever you choose, you can reduce the work by thoroughly sweeping first. Get as much of the grime out of there dry, before you start using water and detergent.

If you haven't used a power washer before, be careful. There is a learning curve to using them and you can damage even concrete with water pressure.

Wet the area and use an over-the-counter degreaser, such as ORANGE CITRUS concentrate. Allow the degreaser to sit on the spot for a few hours. Scrub with a scrub brush and rinse.

Check in auto supply areas of stores such as Wal-Mart, they also have degreasers.

An electric floor scrub machine using brushes with scrubbing pads works well after degreaser has been left on the floor for a couple hours. Rinse garage using a garden hose.

Kitty litter will soak up liquids nicely but it won't clean a dirty floor. Just spread it on the oil and mush it around until it's saturated, then scoop it off with a dust pan. Then put more on and scrub it hard using a block of wood, perhaps an 8" section of 2x4. Then sweep it up.