The OCS + OH reaction occurs either via adduct formation or direct S-abstraction. We investigate OH-oxidation of OCS using quantum chemical methods and find that the OC(OH)S adduct reacts rapidly with O2 forming SOOH + CO2. SOOH rapidly dissociates under atmospheric conditions regenerating OH. We interpret earlier experimental results based on monitoring OH-loss, and find that OH-regeneration in presence of O2 may explain the insensitivity of the reaction rate to pressure and O2. We calculate a rate constant of 3.52×10-163.52×10-16 cm3 s−1 at 10 Torr increasing to 7.20×10-167.20×10-16 cm3 s−1 at 700 Torr. In addition we present a new experimental determination of the OCS + OH rate constant of (5.3±3.6)×10-15(5.3±3.6)×10-15 cm3 s−1 at 296 K and 700 Torr using relative-rate technique