The reactions of oxygen atoms with the CHX2 (X = F, Cl) radicals have been studied for the first time using the G2(MP2) method. The extensive calculations reveal the capture-limited association-elimination mechanism for the O + CHX2 reaction. For the O(P-3) + CHF2 reaction, two energy-rich intermediates, CHF2O* and CF2OH*, are formed along the reaction path. CHF2O* has four unimolecular production channels: H + CF2O; CF2OH; F + CHFO; and HF + FCO. The isomer CF2OH Can decompose to H + CF2O, HF + FCO, OH + CF2, and F + FCOH. The analogous production channels such as Cl + CHClO, H + CCl2O, HCl + ClCO, OH + CCl2, and Cl + ClCOH are investigated for the O(P-3) + CHCl2 reaction. It should be noted that three-center direct elimination of HCl from the adduct CHCl2O* was characterized using the semiempirical AM1 method. On the basis of the ab initio potential energy surfaces, the kinetics and the dynamics for these two atom/radical reactions are discussed qualitatively. For the O + CHF2 reaction, H and CF2O are predicted to be the major products. For the O(P-3) + CHCl2 reaction, the major production channel is the formation of Cl + CHClO, while the HCl + ClCO channel may be competitive.