The time evolution of oxygen plasma treated polystyrene (PS) surfaces was investigated upon storing them in the air under controlled humidity conditions. The methods of water contact angle, X-ray photoelectron spectroscopy (XPS), sum frequency generation (SFG) vibrational spectroscopy, and atomic force microscopy (AFM) were used to infer the surface properties and structure. Chemical groups containing oxygen were formed on the PS surface with the plasma treatment, demonstrated by water contact angle and XPS. The surface polarity decayed markedly on time, as assessed by steady increase in the water contact angle as a function of storage time, from zero to around 60°. The observed decay is interpreted as arising from surface rearrangement processes to burying polar groups away from the uppermost layer of the surfaces, which is in contact with air. On the other hand, XPS results show that the chemical composition in the first 3 nm surface layer is unaffected by the surface aging, and the depth profile of oxygen is essentially the same with time. A possible change of PS surface roughness was examined by AFM, and it showed that the increase of water contact angle during surface aging could not be attributed to surface roughness. Thus, it is concluded that surface aging is attributable to surface reorganization and the motion of oxygen containing groups is confined within the XPS probing depth. SFG spectroscopy, which is intrinsically interface-specific, was used to detect the chemical structure of PS surface at the molecular level after various aging times. The results are interpreted as follows. During the aging of the plasma treated PS surfaces, the oxygen containing groups undergo reorientation processes toward the polymer bulk and/or parallel to the surface, while the CH2 moiety stands up on the PS surface. Our results indicate that the surface configuration changes do not require large length scale segmental motions or migration of macromolecules. Motions that are responsible for surface configuration changes could be relatively small rotational motions. The aging behaviors under different relative humidity conditions were shown to be similar from 18% to 91%, whereas the kinetics of surface polarity decays were faster in higher relative humidity. Here, the surface rearrangement of polystyrene films that were previously treated by oxygen plasma and aged, and was investigated in terms of contact angle after the water immersion. The contact angles of the water-immersed samples were found to change and approach the initial values before the immersion asymptotically.