Answer:
The density of oil is less than the density of water (which is about 1.0 g/mL). So the two liquids will stratify and create layers based on their comparative densities. Since oil is less dense than water, oil will rise to the surface and can easily be taken out.
To test this, you could put water in a bowl or a pot, then put in some cooking oil (it doesn't matter what kind). You will see for yourself that the oil remains on the surface of the water, because it is less dense.