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