The disposition of the rovibrational levels in a diatomic molecule has a major influence on the outcome of inelastic collisions involving that molecule. In the case of the hydrides of moderately heavy elements, unusual collisional properties are anticipated in view of conflict between the demands of momentum and energy in these species. This arises because hydride rotational and vibrational quanta are generally large yet the species may be quite heavy and thus carry substantial momentum. This leads to competition between the momentum based mechanism for elastic transfer and constraints which result from energy conservation. We illustrate these principles in investigating rotational, vibration-rotation, and quasiresonant vibration-rotation transfer, as well as vibrational predissociation of OH-containing van der Waals molecules. Collisional transfer is (almost) invariably constrained by energy conservation in this species and the impact of this on the linear-to-angular momentum mechanism is strongly evident in the collisional behavior of the OH molecule. Molecular collision partners may accept vibrational energy from OH without generating angular momentum, resulting in more efficient deactivation of vibrationally excited OH. Recent observation of emission from very high N levels of (X)(2)Pi OH in the nightglow appears to represent only the second recorded example of quasiresonant vibration-rotation transfer.