The depolarization of OH (X2Pi3/2,v=0,J=1.5-6.5,e) rotational angular momentum (RAM) in collisions with He and Ar under thermal conditions (298 K) has been studied using two-color polarization spectroscopy (PS). Orientation or alignment of the OH RAM was achieved using circularly or linearly polarized pulsed excitation, respectively, on the off-diagonal OH A2+-X2Pi (1,0) band. The evolution of the ground-state OH (X) RAM polarization, exclusively, was probed using an independent, linearly polarized pulse tuned to the diagonal OH A 2+-X2Pi (0,0) band. The PS signal decay rate constant kPS decreases with increasing rotational quantum number for OH (X) +Ar but does not vary monotonically for OH (X) +He. The measured kPS equals the sum kRETRET + k? + kdep, where k RETRET + k? + kdep are the rate constants for rotational energy transfer, ? -doublet changing collisions, and rotationally elastic depolarization (of orientation or alignment of the OH (X) angular momentum, as specified), respectively. Values of kdep can be extracted from the measured kPS with prior knowledge of kRET and k?. Because kRET and k? were not previously available for collisions of Ar with OH (X, v=0), we performed exact, fully quantum-mechanical scattering calculations on a new potential energy surface (PES) presented here for the first time. The raw experimental results show that kdep is systematically markedly higher for alignment than for orientation for OH (X) +Ar but much more weakly so for OH (X) +He. Calculated kRET and k? values at 298.15 K are consistent with a substantial contribution from kdep for OH (X) +Ar but not for OH (X) +He. This may point to the role of attractive forces in elastic depolarization. The experimental results provide a very sensitive test of the ability of the most recent ab initio OH (X) -He PES of Lee [J. Chem. Phys. 113, 5736 (2000)] to reproduce kRET + k? accurately. © 2008 American Institute of Physics.