Spectral-type distribution of the O-type stars in our sample, binned per spectral subtype (SpT). Different colors and shadings indicate different luminosity classes (LC); see legend. The legend also gives the total number of stars in each LC class (e.g., 40 LC III). In parentheses we provide the number of stars that were given an ambiguous LC classification within each category in Walborn et al. (2014) (e.g., 3 in LC III). They are plotted in their corresponding category with lower opacity (see main text for details).