Representative spectral windows for selected O9-B0 stars. Each panel compares the observed spectra with the tailored FASTWIND models calculated for the final parameters derived from the Bayesian analysis. Multiple stars are shown in each panel (vertically shifted for clarity). The simultaneous agreement across H, He, and metal lines demonstrates the robustness of the inferred parameters for the late O/early B stars. Due to the possible effect of the magnetic field, the Hα core was excluded from the evaluation of the model likelihood for HD 149438.