Variational calculations of excited vibrational states for the OCS molecule, using generalized internal coordinates properly optimized, are presented. The calculations are made for two empirical and one ab initio potential energy surfaces previously reported. It is shown that the computed vibrational frequencies differ considerably from the experimental values for the three potential surfaces employed. Consequently a new and much more accurate potential surface is determined for OCS by nonlinear least-squares fitting to the observed vibrational terms. The surface is expressed as a Morse-cosine expansion in valence coordinates and its quality is checked by computing the vibrational frequencies of three isotopic species of the molecule.