Association of ground-state oxygen and hydrogen atoms at pressures of 0.4–4.0 torr and 300°K results in chemiluminescence. The emission corresponds to the transition OH(A2σ+)→OH(X2Π). The (0,0) band is the most intense but the (0,1)(1,0) and (1,1) bands were also observed. The (0,0) band intensity was found to follow the kinetic relation I=K[O][H], where K decreases with pressure. The kinetics are consistent with a two-body excitation followed by collisional quenching. Since the (A2σ+) state does not correlate with ground-state atoms it must be excited by preassociation. Evidence for such a curve crossing near the heat of association is obtained from the vibrational intensity ratios and from the rotational energy distribution.