According to Dick, “One osmole is defined as the osmotic pressure of a 1.0 molal solution of an ideal non-electrolyte.”
Dick DAT. Osmotic properties of living cells. Int Rev Cytol 1959;8:387-448. Article Osmotic Properties of Living Cells
See also the following review here:
Sweeney TE, Beuchat CA. Limitations of methods of osmometry: measuring the osmolality of biological fluids. Am J Physiol 1993;264(3 Pt 2):R469-80. https://www.researchgate.net/profile/Terrence_Sweeney/publication/14742310_Limitations_of_methods_of_osmometry_Measuring_the_osmolality_of_biological_fluids/links/574db83b08ae82d2c6be21ef.pdf