A consistent analysis, based on non-LTE codes with sphericity and mass loss,of different ionization species in the far-UV and UV spectra of O-type starsallowed us to constrain the long-time uncertain ionization equilibrium in thestellar winds, to quantify the role of soft X-rays from shocks and to reconcileotherwise discrepant UV line diagnostics. The modeling initially performed withthe WM-basic code led to a revised (downwards) Teff scale for Galactic O-typestars, a result with important implications for the energy-balance calculationsin HII regions and for massive star evolution. A further analysis using theCMFGEN code, including clumping effects, allowed us to resolve the remaining discrepancies in the modeling of UV lines, in particular the PV line in the far-UV, and to reconcile mass loss and photospheric parameters derived from opticaland UV lines.