Notes. ^†If not mentioned otherwise, the works used to calculate the numbers of observed stars are the same as the ones quoted in Fig. 1. ^(b)Stars with spectral type Of/WN were excluded from this number, as these are not covered by our τ-criterion (see Appendix C). ^(c)Only 7 of the WC/WO stars have been analyzed with non-local thermodynamic equilibrium stellar atmosphere models, but 26 are confirmed (Breysacher 1981; Testor et al. 1993; Barlow & Hummer 1982; Neugent et al. 2012). ^(d)Doran et al. (2013) investigated the completeness of the O-star content in the 30 Dor region of the LMC and reported the presence of about N_{O,  30Dor} = 570 O stars. Furthermore, we know that 30 Dor hosts N_{WR,  30Dor} = 25 WN/WC/WO stars (Doran et al. 2013, and references therein). Assuming that WR stars (excluding the extremely young Of/WN stars) are tracers of massive star populations, we infer that the number of O-stars in the LMC can be approximated as N_O,  LMC = N_WR,  LMC/N_{WR,  30Dor} ⋅ N_{O,  30Dor}. The number quoted here corresponds to the average of this range. ^(e)The BLOeM sample, i.e., the largest spectroscopic sample of massive stars in the SMC, contains 159 O-stars and has an estimated completeness of around ∼35% compared to Gaia DR3. This implies that there are ∼500 O-type stars in the SMC (Shenar et al. 2024). Note that this number might be larger since Gaia cannot resolve the dense cores of young clusters.