Spectroscopic HR diagram indicating the location of the 285 Galactic O-type stars investigated in this work. Different colors and symbols are used to highlight luminosity classes. Individual uncertainties are included as error bars. Evolutionary tracks and position of the ZAMS from the non-rotating, solar-metallicity models by Ekström et al. (2012), plus the log g = 3.7 dex line of constant gravity, are also included for reference purposes. The gray shadowed area highlights a region of the diagram which is void of stars (see main text and Holgado et al. 2020, for further explanations).