log(f_X/f_bol) relation for O stars of all luminosity classes and as a function of the spectral subtype (color coded). Known spectroscopic binaries are highlighted with a ring around the solid circle and magnetic O stars are shown with an asterisk. In the top panel the log(f_X/f_bol) relation in the soft band (0.5–2.5 keV) is shown, while in the bottom panel the same is shown in hard X-ray band (2.5–12 keV).