Reactions of the hydrogen atom and the oxygen molecule are among the most important ones in the hydrogen and hydrocarbon oxidation mechanisms, including combustion in a supercritical CO2 (sCO(2)) environment, known as oxy-combustion or the Allam cycle. Development of these energy technologies requires understanding of chemical kinetics of H + O-2 (sic) HO + O and H + O-2 (sic) HO2 in high pressures and concentrations of CO2. Here, we combine quantum treatment of the reaction system by the transition state theory with classical molecular dynamics simulation and the multistate empirical valence bonding method to treat environmental effects. Potential of mean force in the sCO(2) solvent at various temperatures 1000-2000 K and pressures 100-400 atm was obtained. The reaction rate for H + O-2 (sic) HO + O was found to be pressure-independent and described by the extended Arrhenius equation 4.23 x 10(-7) T-0.73 exp(-21 855.2 cal/mol/RT) cm(3)/molecule/s, while the reaction rate H + O-2 (sic) HO2 is pressure-dependent and can be expressed as 5.22 x 10(-2) T-2.86 exp(-7247.4 cal/mol/RT) cm(3)/molecule/s at 300 atm.