Distribution in a spectroscopic HR diagram of the sample of Galactic O-type stars considered in this work (large filled symbols). For reference purposes, the full analyzed IACOB sample of O-type stars is included as small size circles, while red and yellow symbols are for LC I and II. The GENEC evolutionary tracks during the main sequence are plotted for v_ini/v_crit = 0 with solid lines. Dotted lines are used for gravities decreasing from log g = 4.2 dex (left) in steps of Δ log g = 0.2 dex.