Comparison of one of our representative O 3 III spectra (i.e., the TWUIN T-8 model, see Table 1) to an O 3 synthetic star from the literature. Both stars are placed at the same distance of 10 pc. An effect of line broadening due to fast rotation is taken into account, as described in Sect. 4 assuming the same rotational velocity, which corresponds to the T-8 model. See Sect. 6.1 for more details.