  We present observations and analyses of seven Galactic O stars of type O6 and
earlier. Analyses are carried out using NLTE plane-parallel, hydrostatic models
as well as NLTE spherical models with mass-loss. With detailed calculations for
the former and simulations for the latter, it is shown that the flux blocking
due to UV metal lines is important for these objects and the way the mechanism
operates is explained. We find that the plane-parallel, hydrostatic unblanketed
model atmospheres have increasing difficulties in fitting the early-type
spectra of massive stars, and for 50000 K and above a fit seems to be
impossible. We then perform an analysis using spherical models with mass-loss.
Gravities should be increased by 0.1-0.25 dex, reducing, but not solving, the
mass discrepancy. We show that spectroscopic masses are in better agreement
with the theory of radiatively driven winds than evolutionary masses are. A
helium abundance larger than solar is obtained for most objects. Some
additional effects (partly related to present approximations) that have an
influence in our analyses are studied. Some stars of our sample have such high
mass-loss rates that the derivation of gravities from the wings of Balmer
lines, in particular Hgamma, becomes doubtful.
  From the point of view of individual stars, we have analysed some of the most
massive and luminous stars in the Milky Way. According to our analysis, three
of them (Cyg OB2-7, HD 15570 and HD15558) have particularly large initial
masses, close to or in excess of 100 Msun. HD 5689 could have been erroneously
assigned to Cas OB7 and might be a runaway star.
