  We are now in an era of large spectroscopic surveys of OB-type stars.
Quantitative spectroscopic analysis of these modern datasets is enabling us to
review the physical properties of blue massive stars with robust samples, not
only revisiting mean properties and general trends, but also incorporating
information about the effects of second-order parameters.
  We investigate the spectral type -- effective temperature (SpT - Teff)
calibration for O-type dwarfs, and its claimed dependence on metallicity, using
statistically-meaningful samples of stars extracted from the IACOB and VFTS
surveys.
  We perform a homogeneous differential spectroscopic analysis of 33 Galactic
and 53 LMC O~dwarfs (spanning spectral types of O4 -- O9.7) using the
IACOB-GBAT package, a $\chi^2$-fitting algorithm based on a large pre-computed
grid of FASTWIND models, and standard techniques for the hydrogen/helium
analysis of O-type stars. We compare the estimated effective temperatures and
gravities as a function of (internally consistent) spectral classifications.
  While the general trend is that the temperature of a star increases with
earlier spectral types and decreasing metallicity, we show that the large range
of gravities found for O-type dwarfs -- spaning up to 0.45-0.50 dex in some
spectral bins -- plays a critical role on the dependence of the effective
temperature calibrations as a function of spectral type and metallicity.
  This result warns us about the use of SpT - Teff calibrations for O-dwarfs
which ignore the effects of gravity, and highlights the risks of employing
calibrations based on small samples. The effects of this scatter in gravities
(evolutionary status) for O-type dwarfs should be included in future recipes
which employ SpT - Teff calibrations.
