We present a new vibrationally averaged He-OCS potential energy surface that is obtained from a combination of Moller-Plesset perturbation theory for the helium-molecule interaction and coupled cluster theory for the intramolecular vibrational potential. Employing this potential in quantum Monte Carlo calculations for He-N-OCS complexes shows a blueshift of the OCS vibration for small N that is followed by a transition to a redshift for larger N. The size dependence of the vibrational shift is in good agreement with recent experimental measurements. We then make a comparative study of the effective rotational spectroscopic constants B-eff and D-eff calculated for small N values with this vibrationally averaged potential, with the corresponding values obtained from three previous He-OCS potentials. We find that the vibrationally averaged potential provides the most accurate description of the spectroscopic constants over the size range N=1-8 for which experimental data are available. We rationalize this improved description in terms of the detailed differences in the secondary minimum and saddle point regions of the underlying He-OCS interaction potential, in addition to the behavior at the lowest potential minimum. This analysis indicates that the spectroscopy of complexes with N>1 provides valuable information on the shape of the potential energy surface in regions that are not accessed by the N=1 He-OCS complex, but that are important for understanding the molecular spectroscopy in larger complexes and in droplets. (C) 2004 American Institute of Physics.