Stop the car! This is one warning light that isn't joking. The oil light comes on when your engine suffers a drop in oil pressure. Without oil pressure it can't lubricate itself, and the result is self-destruction, meaning you have to do some seriously expensive internal engine repairs.
If the oil light comes on when you're on the road, you should pull over as soon as it's safe and turn the engine off. While you're on the side of the road, you should check the oil. If it's low, go ahead and add some engine oil and see if it goes off. If not, it's time to take it to the shop.