Doppler dimming of the O VI resonance lines (λ1032Å, λ1037Å) in an expanding corona is calculated including the pumping effect on the O VI λ1037.61 Å of both C II lines at λ1036.34Å and λ1037.02Å, and the effect of the width of the absorption profiles of the coronal oxygen ions along the incident radiation. The pumping effect of the C II line at λ1036.34Å allows us to extend to approximately 450kms−1 the measurement of solar wind velocities with the O VI line ratio technique. Since the emissivity ratio of the O VI doublet depends on the width of the oxygen coronal absorbing profiles, this ratio can provide an accurate measurement of the solar wind velocity in the case that the width of the absorbing profile along the direction of the incident radiation is independently determined. However, if on the one hand the ratio of the emissivities of the O VI doublet has limitations in probing the wind velocity, on the other hand it can be used as a diagnostics for inferring the velocity distribution of the coronal O VI ions along the radial, and detecting possible velocity anisotropies. This diagnostics, applied to recent observational results, allows us to infer that the velocity distribution of the oxygen ions is much broader in the direction perpendicular to the magnetic field direction, and that the acceleration of the fast solar wind in the first 2 solar radii is high.