The reaction of oxygen atom with chlorinated methyl radical has been studied using the G2MP2 level of theory. The computation reveals an association-elimination mechanism. The addition reaction of O((3)p) With CH2Cl proceeds to the formation of an energy-rich intermediate OCH2Cl*. Four product channels of OCH2Cl* are found: H + CHClO, Cl + CH2O, CHClOH, and H-2 + ClCO. The isomer CHClOH also has four product channels: H + CHClO, HCl + HCO, H-2 + ClCO, and OH + CHCl. On the basis of this ab initio potential energy surface and the experimental rate constant at 297 K, the kinetics of the title reaction have been calculated by a "loose transition-state" model and RRKM theory. The overall rate constants can be fitted to a double-exponential formula k(T) = (1.51 +/- 0.01) x 10(-10) exp[(1.36 +/- 0.01) kJ mol(-1)/(RT)] + (2.23 +/- 0.09) x 10(-10) exp[(-27.48 +/- 0.73) kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1) over the temperature range 250-2000 K. The energy-specific rate constants of the unimolecular decomposition of the activated adduct OCH2Cl* are used to estimate the branching ratio. The production of H + CHClO is the major reaction channel, and the production of Cl + CH2O is the minor reaction channel.