Time-resolved infrared emission of CO2 and OCS was observed in reactions O(3P) + OCS and O(1D) + OCS with a step-scan Fourier transform spectrometer. The CO2 emission involves δv3 =-1 transitions from highly vibrationally excited states, whereas emission of OCS is mainly from the transition (0, 0°, 1) → (0, 0°, 0); the latter derives its energy via near-resonant V-V energy transfer from highly excited CO2. Rotationally resolved emission lines of CO (V ≤ 4 and J ≤ 30) were also observed in the reaction O(1D) + OCS. For O(3P) + OCS, weak emission of CO2 diminishes when Ar is added, indicating that O(3P) is translationally hot to overcome the barrier for CO2 formation. The band contour of CO 2 agrees with a band shape simulated on the basis of a Dunham expansion model of CO2; the average vibrational energy of CO 2 in this channel is 49% of the available energy. This vibrational distribution fits with that estimated through a statistical partitioning of energy E*= 18 000 ± 500 cm-1 into all vibrational modes of CO2. For the reaction of O(1D) + OCS, approximately 51% of the available energy is converted into vibrational energy of CO2, and a statistical prediction using E*= 30 000 ± 500 cm-1 best fits the data. The mechanisms of these reactions are also investigated with the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) method. The results indicate that the triplet O(3P) + OCS(X1σ+) surface proceeds via direct abstraction and substitution channels with barriers of 27.6 and 36.4 kJ mol-1, respectively, to produce SO(X3σ-) + CO(X1Ó+) and S(3P) + CO2(X 1A1), whereas two intermediates, OSCO and SC(O)O, are formed from the singlet O(1D) + OCS(X1σ+) surface without barrier, followed by decomposition to SO(a1.) + CO(X 1σ+) and S(1D) + CO2(X 1A1), respectively. For the ground-state reaction O( 3P) + OCS(X1σ+), the singlet-triplet curve crossings play important roles in the observed kinetics and chemiluminescence.