Finally, Fullerton et al. (1996) have conducted a spectroscopic survey of O stars and observed intrinsic absorption line profile variability (LPVs) for about 77% of their sample. They report an increase of incidence and amplitude of variability with increasing stellar radius and luminosity, as well as no statistically significant line profile variability for dwarfs earlier than O7. While Fullerton et al. attempt to relate their findings to the predictions of strange-mode pulsation in O stars by (see their Fig. 13) with the occurrence of sub-surface convection as depicted in Fig. \ref{fig:hrdvel} indicates the possibility of a physical connection between line profile variability and sub-surface convection in O stars.