Oil and water do not mix because the molecules in water are polar, which causes them to be attracted to other water molecules and exclude the oil molecules. When the oil molecules are excluded from the water molecules, they clump together with one another.
Water and oil are immiscible, which means they will not mix. Liquids are said to be immiscible when the molecules of the same liquid have a greater force of attraction than they do with the molecules of another substance. The liquid that has a higher density will sink to the bottom of the solution, and a liquid that is less dense will rise to the top.