Force field molecular dynamics simulations of aqueous solutions of TMAO and urea are used to investigate the delicate interactions in binary and ternary mixtures of one of the most important osmolyte systems. We explore the effect of the choice of force fields on local interactions and thermodynamics. Fully decomposed dielectric relaxation spectra from simulations are used to interpret existing experimental data and evaluate currently used fitting techniques. We show that many force field combinations describe the potential of mean force between urea and TMAO, but it is more challenging to describe thermodynamic data for the ternary system like activity coefficients.