OLI uses a standard approach to calculate osmotic pressure based on the activity of water.
\[ \Pi = -\frac{RT \ln a_{H_2O}}{\bar{V}_{H_2O}} \]
Where
π= osmotic pressure
R = the universal gas constant
T = temperature
aH2O = the activity of water at the temperature T
V̅ = the partial molal volume of water at temperature T