We present a full-dimensional potential energy surface for the OCS-H₂ system that includes explicit dependence on the Q₃ normal vibration of the OCS molecule. From the integration of this potential over the Q₃ coordinate two vibrationally adiabatic surfaces are generated. These are then used to compute the rovibrational energies for the OCS-H₂ complexes with the OCS molecule in both its ground and first excited vibrational states and for all H₂ isotopomers. Comparison with the available experimental data shows an overall good agreement for all spectroscopic parameters. The full potential energy surface is also integrated over the H₂ angular variables to obtain a reduced dimensionality effective potential that describes the interaction between an OCS molecule and a spherical H₂ molecule. The accuracy of this approximation, widely employed in Monte Carlo simulations of H₂ clusters, is then analysed here in the case of the OCS-H₂ complexes by comparing the rovibrational energies computed with the present potential interaction in both full and reduced dimensionality.