Effective temperature vs. spectral subtype but now displaying fits that combine our sample with that of Mokiem et al. (2007b): CS = Combined sample (see main text). Stars with spectral subtype O2-3 or later than O9 have been plotted with lower opacity. The dashed lines give the theoretical calibrations of Martins et al. (2005) for Galactic class III and I stars. The leading number in the legend refers to the total number of O3-O9 stars for which the fit has been derived. The trailing number refers to the total number of stars in each sample.