Oil is less dense than water. This can be seen when water is poured into a container with oil, in which case the water sinks to the bottom, and the oil stays on top.
Since oil and water have such different densities, they do not mix. If a container with equal quantities of oil and water is shaken, the oil breaks up into little droplets rather than mixing with the water. When an oil spill occurs in the ocean, the oil always floats on top in patches. In the case of a fire, it is a bad idea to throw water on burning oil or grease. Rather than mix and dilute the oil, the water can make the fire worse.