It's kind of complicated. There is no "sucking" force.
By definition the osmotic pressure is the magnitude the hydrostatic pressure must have to oppose net movement of water. It's easier (for me at least) to think of osmotic pressure not as any force pushing water around, but as a measure of how strong the hydrostatic pressure on one side needs to be to cancel out the favorable energy change of having water flow to the more hypertonic side.
For a J-tube with a semipermeable membrane at the bottom, when you add salt to one side of two equal volumes of water, the water exhibits net flow to the side with the salt. This happens because it is a thermodynamically favorable process; it allows for greater solvation of the salt and an increase in entropy. However, it also increases the hydrostatic pressure on that side, and decreases the hydrostatic pressure on the hypotonic side. At some point, the hydrostatic pressure (which remember, is just the pressure of the fluid on that side against the membrane due to its own weight) is so great and the h. pressure on the hypotonic side is so low that the thermodynamic favorability of osmosis is no longer sufficient to overcome to deficiency in force pushing the water into the hypertonic side. The hydrostatic pressure of the hypertonic side at this point, the point where osmosis stops, is the osmotic pressure.
