We present differential angular cross sections for O(P-3)+Ar(S-1) scattering at collision energies near 90 kcal mol(-1) (similar to8 km s(-1) relative velocity) from molecular beam measurements and high-level theoretical calculations. Beams of hyperthermal O(P-3) are now being used to investigate novel gas-phase and gas-surface chemistries, and the comparison of theory and measurements on this simple system will be a stringent test of the experimental methodology. Potential energy curves were generated for O(P-3)+Ar(S-1) using a large cc-pVQZ basis within a valence multi-configuration plus perturbation theory treatment. These curves were then used in quantum scattering calculations to generate differential cross sections. Agreement between experiment and theory is excellent. In addition to these comparisons, the cross sections were used in direct simulation Monte Carlo calculations to investigate effects of increasing the Ar flux above the "single-collision" regime. As the Ar flux increases, the observed differential angular cross sections change in two ways. In addition to the main "single-scatter" peak along the incident O-atom beam direction, a secondary O-atom peak appears in the direction of the incident Ar beam, and the multiple-scattered O-atom translational energy starts to reflect the energy of the relatively slow moving Ar beam. (C) 2004 American Institute of Physics.