  We have undertaken quantitative analysis of four LMC and SMC O4--9.7 extreme
supergiants using far-ultraviolet FUSE, ultraviolet IUE/HSTand optical VLT UVES
spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent
treatment of line blanketing are used to analyse wind & photospheric spectral
features simultaneously. Using Halpha to constrain dM/dt, HeI-II photospheric
lines reveal T_eff's which are systematically (5-7.5kK) & substantially
(15-20%) lower than previously derived from unblanketed, plane-parallel,
non-LTE photospheric studies. We have confidence in these revisions, since
derived T_eff's generally yield consistent fits across the entire 912-7000Ang
observed spectral range. In particular, we are able to resolve previous
UV-optical T_eff discrepancies. `Of' classification criteria are directly
linked to (strong) nitrogen enrichment (via NIII 4097) & (weak) carbon
depletion (via CIII 4647-51), providing evidence for mixing of unprocessed and
CNO processed material at their stellar surfaces. Oxygen abundances are more
difficult to constrain, except via OII lines in the O9.7 supergiant for which
it is also found to be somewhat depleted. The effect of wind clumping is also
investigated, for which PV 1118-28 potentially provides a useful diagnostic in
O-star winds. Revised stellar properties affect existing calibrations of (i)
Lyman continuum photons - a factor of two lower for the O4 supergiant; and (ii)
kinetic energy released into the ISM by O supergiants. Our results also have
importance for the calibration of the wind momentum-luminosity relationship for
OB stars.
