Locations of only the identified O-type stars in Gaia DR2 (red circles): panel a) with ±0.1 mas systematic errors added in quadrature to the formal DR2 errors; only stars with corresponding absolute distance uncertainties <0.2 kpc are shown; Panel b) with ±0.05 mas systematic errors added in quadrature to the formal DR2 errors; only stars with corresponding absolute distance uncertainties <0.2 kpc are shown. See Fig. 1 caption for other details, but note that the 10% error threshold in that figure is not used here.