Early O-type stars are incredibly rare from an IMF perspective but compensate with the sheer magnitude of their feedback. In highly star-forming regions, a handful of the earliest O stars can provide 50% of the of radiative and mechanical feedback in their host galaxies. Massive stars are progenitors of supernovae (SN), black holes, gamma-ray bursts (GRB) and gravitational wave events, are the source of heavy elements essential for life and are our closest proxies to the first generations of stars. The combination of all of these factors establishes massive stars as some of the most impactful astrophysical objects. In order to gain a complete understanding of these stars we must be able to quantify the key parameters. Perhaps the most important property is the stellar mass, the evolution of which remains highly uncertain due to mass loss through stellar winds.