Oil and water do not mix because the molecules (tiny particles) of which they are composed are so different. The molecules in oil are much bigger and contain many more atoms than those of water.
When different liquids mix, it is because they have similar types of molecules which readily link up with each other, like milk and water. In the case of oil and water the groups of molecules prefer to stay apart.
The patches of oil floating on top of the water are usually circular because of another characteristic of molecules, which produces what scientists term surface tension. This is a cohesive force caused by the attraction of the molecules to each other. They cling so tightly that they produce a surface layer which acts like an elastic skin or the rubber envelope of a balloon. The molecules are trying to pull the liquid into as small a space as possible. As well as producing a circular shape, this tension makes the surface area of each oil patch as small as possible.