So, if we have any fluid moving along the following conduit, then the liquid molecules exert a force on the walls of that conduit, as shown in this diagram. And if we take the sum of all these forces, if we find the net force due to all the liquid molecules and divided by the surface area of the wall of the conduit on which those forces act on, then we get the hydrostatic pressure. So notice these two pressures are not exactly the same thing. Osmotic pressure is simply the pressure we have to apply to prevent osmosis from actually taking place, to prevent the water from moving down its osmotic potential gradient. But hydrostatic pressure is the pressure that is a result of that movement of the fluid, the liquid, along some given conduit. It's the pressure that is created by the molecules on the walls of that conduit.