While recent OSN research efforts mostly focused on the synthesis and fabrication of solvent resistant membranes, fundamental understanding of chemical-physical aspects that govern solvent and solute transport in OSN membranes remains entirely unexplored. In spite of this, development of competitive membranes processes relies on the solid molecular-level understanding of transport mechanism in the membrane material. In this talk we critically discuss the hypothesis of membrane compaction, which has been invoked to explain solvent flux vs. p non-linearity in OSN experiments. Although physically sound, occurrence of membrane compaction under pressure does not have an experimental support. To demonstrate that the molecular origin of flux non-linearity is purely thermodynamic, we propose a thermodynamic-diffusion framework which describes solvent transport in OSN membranes in terms of the concentration gradient produced by the applied pressure across the membrane. Solvent diffusion coefficient in the membrane increases with increasing p, which further confirms that flux decline is not related to membrane compaction. This study demonstrates that the solution-diffusion model, if properly corrected for frame of reference (i.e., convection) and non-ideal effects, provides a satisfactory description of small molecule transport in OSN membranes, without the need to resort to pore-flow or more complicated transport models. In this talk we critically discuss the hypothesis of membrane compaction, which has been invoked to explain solvent flux vs. Dp non-linearity in OSN experiments. Although physically sound, occurrence of membrane compaction under pressure does not have an experimental support. To demonstrate that the molecular origin of flux non-linearity is purely thermodynamic, we propose a thermodynamic-diffusion framework which describes solvent transport in OSN membranes in terms of the concentration gradient produced by the applied pressure across the membrane. Solvent diffusion coefficient in the membrane increases with increasing Dp, which further confirms that flux decline is not related to membrane compaction. The developed framework allows to quantify both frame of reference and non-ideal thermodynamic effects on solvent diffusion coefficients in OSN membranes. This study demonstrates that the solution-diffusion model, if properly corrected for frame of reference (i.e., convection) and non-ideal effects, provides a satisfactory description of small molecule transport in OSN membranes, without the need to resort to pore-flow or more complicated transport models. Advancing fundamental understanding of OSN will lay the foundation for a more mature use of this process, and allow the most effective operative conditions to be set to maximize its productivity and efficiency.