Mechanisms and kinetics of the NCCO + O-2 reaction have been investigated using the extrapolated full coupled cluster theory with the complete basis set limit (FCC/CBS) and multichannel RRKM theory. Energetically, the most favorable reaction route involves the barrierless addition of the oxygen atom to one of the carbon atoms of NCCO and the subsequent isomerization-decomposition via the four-center intermediate and transition state, leading to the final products NCO and CO2. At 298 K, the calculated overall rate constant is strongly pressure-dependent, which is in good agreement with the available experimental values. It is predicted that the high-pressure limit rate constants exhibit negative temperature dependence below 350 K. The dominant products are NCO and CO2 at low pressures (ca. < 10 Torr) and the NCCO(O-2) radical at higher pressures, respectively.