The final electronic states of oxygen atoms formed by dissociative recombination of O-2(+) with cold electrons have been measured by an imaging technique at a heavy-ion storage ring. The P-3, D-1, and S-1 quantum yield for O-2(+) in the vibrational ground level was found to be 0.86, 1.09, and 0.05, respectively. The yield of the S-1 state, which is responsible for the 5577 angstrom green light from upper planetary atmospheres, was more than an order of magnitude larger than the theoretical value used in atmospheric models. The results may help refine the models of the chemistry and dynamics of the ionosphere and provide guidance for further quantum-chemistry calculations.