We present a theoretical model for O5+ (O7+) ions as test particles in the fast solar wind using a previous turbulence-driven four-fluid model to establish the background solar wind, which consists of electrons, protons, alpha particles, and O6+ ions. The O5+ (O7+) ions in our model and the O6+ ions in the background are driven by the same mechanism, say, the resonant cyclotron interaction or an exponential heating addition. The ionization and recombination processes of O ions are taken into account. It is shown that the differential flow speeds between O6+ and O5+ or O7+, which are found to be in the range of 0.3--2, play very different roles in the formation of O charge states. This is due to discrepancy between the freezing-in distances of the two ion species. O7+ forms predominantly below 1.2 RS too close to the Sun to develop a differential streaming between O ions. O5+, on the other hand, freezes-in at about 1.8 RS where differential flows are well developed and therefore important for the formation of O5+ ions.