Osmosis occurs when a semi-permeable membrane, one permeable to water but not to salts or organic molecules in solution, separates water or a dilute solution from a more concentrated solutions.  Water molecules have a stronger tendency to escape from water than from a solution.  The dilution of the salt solution continues until the pressure created by surface level difference equals the osmotic pressure of salt solution.  Generally, osmotic pressure is in proportion to the concentrate of salt solution.