The driving force of osmosis is what is known asosmotic pressure.It is the difference in the pressure between the solution and the solvent system or it is the excess pressure which must be applied to a solution in order to prevent flow of solvent into the solution through the semi-permeable membrane.Once osmosis is complete the pressure exerted by the solution and the solvent on the semi-permeable membrane is same.
Van't Hoff equationfor dilute solutions is ( parallel to ideal gas equation)

V = nRT
where

= Osmotic pressure
V= volume of solution
n= no. of moles of solute that is dissolved
R = Gas constant
T= Absolute temperature