  Massive stars likely played an important role in the reionization of the
Universe, and the formation of the first black holes. Massive stars in
low-metallicity environments in the local Universe are reminiscent of their
high redshift counterparts. In a previous paper, we reported on indications
that the stellar winds of low-metallicity O stars may be stronger than
predicted, which would challenge the current paradigm of massive star
evolution. In this paper, we aim to extend our initial sample of six O stars in
low-metallicity environments by four. We aim to derive their stellar and wind
parameters, and compare these to radiation-driven wind theory and stellar
evolution models. We have obtained intermediate-resolution VLT/X-Shooter
spectra of our sample of stars. We derive the stellar parameters by fitting
synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic
fitting algoritm. We compare our parameters to evolutionary tracks and obtain
evolutionary masses and ages. We also investigate the effective temperature
versus spectral type calibration for SMC and lower metallicities. Finally, we
reassess the wind momentum versus luminosity diagram. The derived parameters of
our target stars indicate stellar masses that reach values of up to 50
$M_{\odot}$. The wind strengths of our stars are, on average, stronger than
predicted from radiation-driven wind theory and reminiscent of stars with an
LMC metallicity. We discuss indications that the iron content of the host
galaxies is higher than originally thought and is instead SMC-like. We find
that the discrepancy with theory is lessened, but remains significant for this
higher metallicity. This may imply that our current understanding of the wind
properties of massive stars, both in the local universe as well as at cosmic
distances, remains incomplete.
