The dynamics of molecules trapped inside or bound to the surface of liquid helium droplets is a most exciting new development. As an aid toward the understanding of the spectrum of OCS in liquid helium, we report the intermolecular potential between He and OCS studied by ab initio calculations and high-resolution microwave spectroscopy. The potential is found to have three minima, corresponding to a T-shaped configuration for the global minimum and secondary minima at either end of the OCS molecule. The three lowest calculated bound states are loosely localized in each of the minima, with the ground state being T-shaped. Ten rotational transitions of the ground state are observed in the frequency region 1.5-45.0 GHz. Comparison of theory with experiment shows good agreement. The agreement improves substantially if the calculated He-OCS intermolecular potential is made uniformly 10% deeper and the He-OCS separation is reduced by 0.05 Angstrom.