The structure of a physisorbed carbonyl sulfide (OCS) monolayer on a well-defined NaCl single crystal surface has been studied by helium atom scattering (HAS), low energy electron diffraction (LEED) and polarized Fourier-transform infrared spectroscopy, in the temperature range 30-100 K. The monolayer growth proceeds via large islands and both HAS and LEED indicate a (2 x 1) commensurate structure with two molecules per unit cell. The binding energy is estimated to be about 0.26 eV from desorption experiments. By contrast, the occurrence of three intense and three weaker absorption peaks in the infrared spectrum of the asymmetric stretching vibration v(3) is difficult to reconcile with the conclusions drawn from HAS and LEED. Therefore semi-empirical potential calculations are performed to determine the monolayer structure. Energy minimization rules out a high molecular density for the layer, and shows the existence of two isoenergetical (2 x 1) and (2 x 2) phases with respectively two and four molecules per unit cell, which differ only by the orientations of the molecules. Because of their similar geometry, these two structures cannot be discriminated by HAS and LEED, but the mixing of these two phases with different vibrational couplings between the admolecules provides a reasonable interpretation of the main features of the infrared spectrum.