We present a two-dimensional quantum-mechanical wave packet study of photoinduced reaction of O-2 on Pt(111) within a mechanism of hot electron/hole excitation of a molecular resonance. Based on three constructed potential energy surfaces including the molecule-surface and intra-molecular coordinates, photoexcitation is simulated by nonadiabatic electronic transitions between the ground state (the chemisorbed O-2(-)) and a negative ion resonance state (the O-2(2-) shape resonance) or a neutral O-2 state. The wave packet dynamics exhibits a fast energy exchange between the two bonds, giving a comparable yield for desorption and dissociation. The calculated branching ratio, BR =0.5-1.0, between desorption and dissociation and the mean kinetic energy of the desorbed molecules (E(kin))/2k(B) = 990 K are comparable with the reported experimental data measured in desorption by nanosecond laser pulses, while the vibrational temperature is first predicted by this calculation. In addition our results indicate the importance of a proper treatment of damping effect in a coherent wave packet after deexcitation.