Molecular dynamics simulations have been performed to clarify the scattering phenomena of oxygen molecules on ionomer thin films, which affect the transport resistance of oxygen in catalyst layers in polymer electrolyte fuel cells. We have evaluated the probability density functions of the translational energy and scattering angle of scattered molecules, and the residence time of oxygen molecules on the ionomer surface. It was found that the energy distributions of scattered oxygen molecules depend on the incident energy and differ from that of thermally equilibrated molecules. On the other hand, the angular distributions are independent of the incident energy, and well reproduced by the diffusive scattering model. These results indicate that oxygen molecules do not accommodate completely with ionomer surface during the collision. We also evaluated the trapping dynamics of oxygen molecules on the ionomer surface in the trajectory calculations. Increasing the normal component of the incident energy results in the longer residence time on the ionomer surface.