The reaction of HCO with O2 has been studied by ab initia molecular orbital and statistical theory calculations. Both the direct abstraction and the association-elimination processes have been considered. The direct abstraction of H by O2 producing the HO2+CO products was found to be unimportant below 2000 K. The association reaction occurs by the attack of O2 at the C atom to form a vibrationally excited complex, HC(O)OO†, which can undergo two reactions. The first possibility is H migration via TS2 forming HOOCO, which rapidly dissociates into either OH+CO2 via TS4 or HO2+CO via TS3; the latter is energetically less favorable. The second possibility is the direct production of HO2+CO from HC(O)OO† via TS5 in a concerted manner. The barrier of TS5 at the G2M level of theory is 23.5 kcal/mol relative to HC(O)OO; this is the major channel for the reaction. Variational transition state theory and Rice-Ramsperger-Kassel-Marcus calculations have been carried out for the direct abstraction and the indirect metathetical mechanisms, respectively. The calculated total rate constant at 1.5 Torr exhibits a small positive activation energy and its absolute values agree closely with experimental data.