De-excitation of metastable O(⁵S) atoms in inelastic two-body collisions with inert gas atoms was observed. The cross section for the ground state product reaction was measured for metastable energies from about 0.5 eV to roughly 10 eV in the laboratory frame. For the heavy noble gases Xe and Kr, the cross section rose steeply from threshold to peak values of about 2-3*10^-17 cm² from where it decreased slightly in the case of Xe and steeply for Kr. Corresponding peak values for Ar were at least a factor of 30 smaller. No inelastic signal could be detected with Ne. The excited state product process involved several excited states for each rare gas. The summed cross sections for these inelastic processes were of the order of 10^-16 cm² for Xe and Kr.