The explicit water molecular dynamics simulation was used to study tetramethyl ammonium and tetraethylammonium chloride and bromide solutions in water at 298 K. The outcome of the simulations in the form of various distribution functions was used to construct the solvent-averaged potentials between interacting molecules. In the next step, which involved the Ornstein-Zernike integral equation theory in the hypernetted chain approximation, these potentials were used to calculate the osmotic coefficients. We showed that this approach is able to explain the experimental results for the osmotic pressure of these salts.