Top panel: uncertainties of the field measurements obtained with the FORS1/2 instruments for various O-type stars. Blue filled circles refer to bright Galactic O-type stars observed with FORS1 (see Bagnulo et al. 2015); red empty squares correspond to LMC/SMC O-type stars observed with FORS2 in Paper I and red filled squares LMC/SMC O-type stars observed with FORS2 and presented in this paper. The solid line is the best hyperbolic fit to the data points. Bottom panel: S/N vs. exposure time. Symbols refer to different observations of O-type stars as explained in the legend. Solid lines show the predictions of ETC version P103.2 for a O5 star observed at airmass =1.5 with grism 1200B through a 1′′ slit width, with 1.0′′ seeing, and fraction of lunar illumination =0.5.