We have investigated the O2 molecular and dissociative adsorption on Ag(111) surfaces. To clarify the adsorption properties, we performed first principles calculation based on spin-polarized density functional theory (DFT) with the generalized gradient approximation for the exchange--correlation energy. We calculated the potential energy surfaces (PES) for thirteen trajectories of O2, which gives us information with respect to the relevant six dimensional PES. From these results, we found that the effective energy barrier of O2 dissociative adsorption is about 1.24 eV. We also found that upon O2 molecular and dissociative adsorption, the magnetic moments per O atom were 0.52 μB and 0.32 μB, respectively. For reference, the isolated O2 and Ag(111) surfaces have the magnetic moments of 2.0 μB and 0.0 μB. In O2 dissociative adsorption, ferromagnetic behaviors were induced by ferromagnetic direct exchange interactions and superexchange interactions. Superlattice structures of O atoms which have an effective ferromagnetic superexchange interaction made ferromagnetic states feasible.