  We aim to constrain the properties and evolutionary status of early and
mid-spectral type supergiants (from O4 to O7.5). These posses the highest
mass-loss rates among the O stars, and exhibit conspicuous wind profiles. Using
the non-LTE wind code CMFGEN, we simultaneously analyzed the FUV-UV and optical
spectral range to determine the photospheric properties and wind parameters. We
derived effective temperatures, luminosities, surface gravities, surface
abundances, mass-loss rates, wind terminal velocities, and clumping filling
factors. The supergiants define a very clear evolutionary sequence, in terms of
ages and masses, from younger and more massive stars to older stars with lower
initial masses. O4 supergiants cluster around the 3 Myr isochrone and are more
massive than 60 Msun, while the O5 to O7.5 stars have masses in the range 50 -
40 Msun and are 4 +/- 0.3 Myr old. The surface chemical composition is typical
of evolved O supergiants (nitrogen-rich, carbon- and oxygen-poor). While the
observed ranges of carbon and nitrogen mass-fractions are compatible with those
expected from evolutionary models for the measured stellar masses, the N/C
ratios as a function of age are inconsistent with the theoretical predictions
for the four earliest (O4 spectral type) stars of the sample. We question the
efficiency of rotational mixing as a function of age for these stars and
suggest that another mechanism may be needed to explain the observed abundance
patterns. Mass-loss rates derived with clumped-models range within a factor of
three of the theoretical mass-loss rates. The corresponding volume-filling
factors associated with small-scale clumping are 0.05 +/- 0.02. Clumping is
found to start close to the photosphere for all but three stars, two of which
are fast rotators.
