The spin-forbidden and spin-allowed reactions of the excited and ground electronic state O(1D, 3P) + N2O(X1Σ+) systems have been studied theoretically. Quantum calculations at the UCCSD(T)/CBS(T, Q, 5)//CCSD/aug-cc-pVTZ level have located two crossing points, MSX1 and MSX2, with energies of 11.2 and 22.7 kcal mol-1 above O(3P) + N2O, respectively. The second-order P-independent rate constants for the adiabatic and non-adiabatic thermal reactions predicted by adiabatic TST/VTST and non-adiabatic TST, respectively, agree closely with the available literature results. The second-order rate constant, k2a = 9.55 × 10-11 exp(-26.09 kcal mol-1/RT) cm3 molecule-1 s-1, for the O(3P) + N2O → 2NO reaction, contributed by both the dominant MSX2 and the minor TS1-a channels, is in reasonable accord with prior experiments and recommendations, covering the temperature range of 1200-4100 K. The calculated rate constant, k2b = 4.47 × 10-12 exp(-12.9 kcal mol-1/RT) cm3 molecule-1 s-1, for the O(3P) + N2O → N2 + O2(a1Δg) reaction, occurring exclusively via MSX1, is also in good agreement with the combined experimental data measured in a shock tube study at T = 1940-3340 K (ref 16) and the result measured by Fourier transform infrared spectroscopy in the temperature range of 988-1083 K (ref 17). Moreover, the spin-allowed rate constants predicted for the singlet-state reactions, k1a = (7.06-7.46) × 10-11 cm3 molecule-1 s-1 for O(1D) + N2O → 2NO and k1b = (4.36-4.66) × 10-11 cm3 molecule-1 s-1 for O(1D) + N2O → N2 + O2(a1Δg) in the temperature range of 200-350 K, agree quantitatively with the experimentally measured data, while the total rate constant k1 = k1a + k1b was also found to be in excellent accordance with many reported values.