Location of 285 likely single and SB1 Galactic O-type stars in the Kiel (right) and spectroscopic HR (left) diagrams. Crosses indicate stars for which we detected clear or likely signatures of spectroscopic binarity. Open symbols, triangles, and squares are stars for which only an upper or lower limit in any (triangles) or both (squares) of the two parameters used to construct these diagrams could be obtained (T_eff and log g_true). Individual uncertainties are included as error bars. Evolutionary tracks and position of the ZAMS (thick solid line) from the nonrotating solar metallicity models by Ekström et al. (2012) and Georgy et al. (2013) are included for reference. The thick solid blue line represents the ZAMS for similar models, but with an initial rotation speed of 40% the critical speed. Isochrones for τ = 1, 2, 3, and 4 Myr are also included. The solid diagonal black line separates the region where no He I lines are available. Last, we also delineate with a red line the region close to the ZAMS where no stars are found.