The flow of water across a membrane in response to differing concentrations of solutes on either side - osmosis - generates a pressure across the membrane called osmotic pressure. Osmotic pressure is defined as the hydrostatic pressure required to stop the flow of water, and thus, osmotic and hydrostatic pressures are, for all intents and purposes, equivalent.
Well, but they're due to different things, aren't they? As you said, osmotic pressure is due to varying ion concentrations across a membrane, but hydrostatic pressure is the actual physical pressure that the fluid exerts on something, like blood pressure. So, in a sense there could be a situation in which they are not equivalent, right? Correct me if I'm wrong because I lost all this info the moment after the MCAT.
I pulled up the info from a reference book, which further states:
Diffusion of water across a membrane - osmosis - generates a pressure called osmotic pressure. If the pressure in the compartment into which water is flowing is raised to the equivalent of the osmotic pressure, movement of water will stop. This pressure is often called hydrostatic ('water-stopping') pressure.