X-ray flux vs. bolometric flux for O stars of all luminosity classes and as a function of the spectral subtype (color-coded with earlier subtypes being bluer, and the later subtypes being redder). Known spectroscopic binaries are highlighted with a ring around the solid circle, magnetic O stars are represented by asterisks, and stars flagged as variable are shown with a squared symbol. The linear fit log(f_X) = a + b × log(f_bol) and the mean log(f_X/f_bol) value are shown as solid and dotted lines, respectively (see text for details).