In this experiment, the volumes are totally irrelevant. This is because all that matters (when regarding immiscible liquids) is density.
Density is the mass of a selected volume of matter (in this case olive oil and water). What is important, is that the water is denser, so it sinks to the bottom.
You can think of it, as an interaction between one volume oil and one volume water. Since the water is denser, the same volume of the stuff must weight more. From here, you can use the same logic as before, the heavier one, the water sinks. If you imagine it this way, on the surface of the two fluids there will always be some of both of them, so they'll arrange themselves in the correct order.
Even if the volumes are not the same, we can use Archimedes' principle, which
indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to theweight of the fluid that the body displaces and it acts in the upward direction at the centre of mass of the displaced fluid. Wikipedia
This ask sounds very technical, but what is means, is that the weight of anything submerged in a liquid is decreased by the weight of the liquid it displaced. To put it in other words, it's weight will decrease by the weight of the displaced liquid. Since these two have the same volume, we can compare only their densities.
Think of the experiment as oil droplets submerged in water. When imagining is this way, each droplet will displace the same amount of water (which is denser), so its weight decrease by more, than it ever was, resulting in a negative weight. Since weight is the force an object exerts on other ones "downwards" to keep it in place. We know that the system will at some point reach equilibrium. When this happens, if there was any oil in the water, it would mean, that it has negative wight, so it has to exert a force upwards to keep it in place. Since this is obviously not true, we know, that at equilibrium state all the water will be below all the oil.