The mechanism for the O + CH2OH reaction was investigated by various ab initio quantum chemistry methods. For the chemical activation mechanism, that is, the addition/elimination path, the couple-cluster methods including CCSD and CCSD(T) were employed with the cc-pVXZ (X = D, T, Q, 5) basis sets. For the abstraction channels, multireference methods including CASSCF, CASPT2, and MRCISD were used with the cc-pVDZ and cc-pVTZ basis sets. It has been shown that the production of H + HCOOH is the major channel in the chemical activation mechanism. The minor channels include HCO + H2O and OH + CH2O. The hydrogen abstraction by an O atom from the CH2OH radical produces either OH + CH2O or OH + HCOH. Moreover, the two abstraction reactions are essentially barrierless processes. The rate constants for the association of 0 with CH2OH have been calculated using the flexible transition state theory. A weak negative temperature dependence of the rate constants is found in the range 250-1000 K. Furthermore, it is estimated that the abstraction processes also play an important role in the O + CH2OH reaction. Additionally, the falloff behavior for the OCH2OH -> H + HCOOH reaction has been investigated. The present theoretical results are compared to the experimental measurements to understand the mechanism and kinetic behavior of the O + CH2OH reaction and the unimolecular reaction of the OCH2OH radical.