Cross sections were determined for collisions of Ar with oriented NO(X (2)Pi), based on full close-coupled calculations and new ab initio potential energy surfaces (PESs). Collisions in which the NO molecules are initially oriented so that the O end preferentially points toward the Ar atom are more effective in promoting spin-orbit changing transitions. The magnitude of the steric asymmetry is consistent with earlier calculations based on a previous PES, and agrees well with experiment. Various modifications of the full PESs were used to explore the origin of the observed features in the steric asymmetries, in particular the striking oscillatory pattern seen in the variation of the steric asymmetry with final state.