SpT- T_eff (middle panels) and SpT-log g_true (bottom panels) calibrations and the 358 O-type stars within the working sample, i.e., with available estimations of T_eff and log g. 234 stars comprising the calibrator sample (solid points) and 124 eliminated (semitransparent) due to qualitycut criteria (Appendix A). Columns divided by LC groups. Dashed lines represent the “observational scales” proposed by M05, while solid lines correspond to the calibrations derived in this work (see definitions in Table B.1). Stars identified as SB1 are highlighted with open symbols. The top panels display the number of stars per SpT bin, where number in brackets (and hatched bars) indicates stars excluded from the calibrator sample (see Sect. 2).