Hexapole state selection of OH molecules and subsequent orientation in an electric field is performed to study orientational effects in rotational excitation of OH in molecular collisions.' Laser-induced fluorescence spectroscopy of OH is used to determine the orientational probability distribution function and to measure the cross sections for excitation. For the collisionally induced transitions of OH in the rotational ground state the steric asymmetry is determined;for collisions with He, Ar, n-H-2, and p-H-2 (n = normal, p = para). The results show that for He excitation is preferential. at the H-end of the molecule, whereas for Ar and Hz a preference for the O-end is shown in transitions to the lowest rotational states.