We present quantitative analyses of 7 galactic O stars of spectral types from O3 to O6. By means of classical hydrostatic and planeparallel models we find the so-called He and Mass Discrepancies for a number of stars of our sample. We also prove the need of line-blocking for the correct reproduction of these spectra. For stars with effective temperature around 50 000 K and above, there is no possibility of reproducing their spectra with these kind of models. Further analyses with unified models, including sphericity and mass loss, allow us to reproduce even the earliest spectra. They also show that the Mass Discrepancy is not only due to the neglect of sphericity and mass loss in the planeparallel analysis, and give us the mass loss rate of these stars. We find that HD15 570, with a present mass of 100 M&sun;, can be the most massive star known in the solar neighbourhood.