Combined experimental and theoretical investigations of the title reactions are presented. Time-resolved Fourier transform infrared (FTIR) emission studies of CO (v = 1) produced from the CH3 + O and CD3 + O reactions show that there is approximately a one-third reduction in the branching to the CO channel upon deuteration of the methyl radical. Direct dynamics, classical trajectory calculations using a B3LYP potential surface, confirm the existence of the CO producing channel. The calculations show that the CO comes from the decomposition of HCO produced by the elimination of H-2 from highly vibrationally excited methoxy radicals. Scans of the potential surface reveal no saddle point for the direct elimination of H-2 from methoxy. The mininum-energy path for this elimination is a stepwise process involving first a CH bond cleavage, forming H + H2CO, followed by an abstraction, forming H-2 + HCO. However, at the high internal energies produced in the initial O + CH3 addition, trajectories for the direct elimination of H-2 from methoxy are observed. The predicted branching ratio between the CO and H2CO channels is in good agreement with previous room temperature measurements, and there is predicted to be little temperature dependence to it. The observed reduction in the branching to the CO channel upon deuteration is also well reproduced in the calculations.