The main concern of this study is to investigate the reaction mechanism of O + COS → products (1). Experiments are conducted by use of an excimer laser photolysis shock tube technique, where mixtures of COS and SO2 diluted in Ar are photolyzed behind reflected shock waves. Time-resolved measurements of O and S atoms are conducted by use of atomic resonance absorption spectroscopy, and the overall rate constant of (1) is determined by the O atom decay at temperatures 1250-1600 K, i.e., k1 = 10-10.18±0.26 exp[-(22.5 ± 7.1) kJ·mol-1/RT] cm3 molecule-1 s-1, which is in good agreement with the former recommendation. It is confirmed in this experiment that the S atom is a direct product of reaction 1. By analysis of time profiles of the S atom, the branching fraction of the S production channel O + COS → S + CO2 (1b), α, is determined against the main channel O + COS → CO + SO (1a), where the S atom consumption reactions, S + SO2 → 2SO (2) and S + COS → S2 + CO (3), are inevitably taken into account. The present experimental result is expressed as α = (0.40 ± 0.10) - (202 ± 137)/T(T= 1120-1540 K). Also, in this kinetic analysis, the rate constant of (2) is simultaneously determined to be k2 = l0-11.01±0.33 exp[-(37.8 ± 8.2) kJ·mol-1/RT]. The reaction mechanism of (1) is examined by comparing the experimental results with those of ab initio potential energy surface/transition state theory calculations.