Classical effects arising from variations in log g and T_eff on optical spectra, as depicted by our grid models. Top panel: SMC models. Lower panel: LMC models. The chosen helium lines are: He I λ4471 and He II 14542. They are often used with other Hei-ii lines to infer T_eff in O stars, usually with a 1-2 kK uncertainty. The surface gravity determination from Hγ (and other Balmer lines) has a typical uncertainty of 0.1-0.2 dex. The effective temperature is fixed in the leftmost panels and the log g is varied (see caption). The other panels have log g fixed and the effective temperature is varied (see caption).