The scattering behaviors of oxygen molecules on a Nafion membrane, which is a typical polymer electrolyte membrane used in polymer electrolyte fuel cells, have been investigated using molecular dynamics simulations. We have evaluated the probability density functions of the translational energy and scattering angle of the scattered oxygen molecules for a wide range of incident conditions and water contents. It was found that the translational energy of oxygen molecules does not accommodate with the Nafion membrane during the collision, and oxygen molecules are reflected diffusely on the surface. Two types of collision behaviors, i.e., single and multiple collisions, were observed in the simulations. Increasing the normal component of the incident energy and the water content results in the longer residence time on the ionomer surface.