Like any stain, oil stains should be removed as soon as possible to prevent any permanent staining to the concrete floors. For new oil stains the first removal attempt should be with dishwashing liquid, soap, and a nylon bristle brush. Although concrete is a tough material, a metal bristled brush should never be used because it could alter the texture of the area you scrubbed, making the stain stand out more than before.
To begin, soak the area covered in oil and the surrounding concrete with water. This will prevent the oil you remove from making a secondary stain. At this time you should also consider where the dirty, oily water from the floor is going. It is irresponsible to let it run into the storm drains.
There are multiple methods for removing oil or grease (both hydrocarbon-based materials) from concrete. Small spills or spots can sometimes be removed with nothing more than a strong detergent, a scrub brush, and a sponge. A more aggressive method is to use a commercial concrete degreaser, a concentrated alkaline soap that's scrubbed into the concrete surface. The soap acts like ball bearings, loosening up the oil to permit easier removal. The downside is that typical degreasers don't actually break down the oil, so they won't work well on concrete that is heavily contaminated or has been contaminated for a long time. Also, they are more effective on porous concrete as opposed to concrete with a hard or dense finish.
Another common treatment for removing oil stains from concrete is a poultice. Used primarily on small, stubborn stains, a poultice is made by saturating an absorptive material (such as kitty litter, pool filter media, or sawdust) with a strong solvent (acetone, xylene, lacquer thinner) and then smearing the material over the stain. Cover the poultice with plastic, and let the process of osmosis take over. The solvent will break down the oil, and the absorptive material will suck it out of the concrete. However, this process takes time and may not be cost-effective or practical for removing large stains.
The most recent advancement in the removal of oil stains from concrete involves using special single-celled microorganisms that thrive on crude oil and its derivatives, eating them up like candy. Enzymes and oxygen digest the oil and turn it into carbon dioxide and more microorganisms. When the food source (oil) is gone, the microorganisms die, leaving the concrete clean and oil-free. This is the same technology used to clean beaches and waterways after large oil spills.