Vibrational excitation to the symmetric stretching, bending and asymmetric stretching modes of OCS molecules by electron and positron impacts is investigated for impact energy from 2 to 6 eV theoretically and experimentally. Because the OCS molecule is a polar molecule, interactions that induce the vibrational excitation for the three modes are primarily long-range permanent dipole interactions. And hence, all excitation cross sections for three modes thus obtained are found to be similar in size both for electron and positron impacts, although for positron, the magnitude is generally slightly smaller by 25% than that of electron. This situation is markedly different from that of the CO2, where for the symmetric stretching mode, the difference between positron and electron impacts is a few orders of magnitude at 5 eV. Therefore, a comparative discussion of CO2 is useful for further understanding. (C) 2000 American Institute of Physics. [S0021-9606(00)00816-3].