The author presents the results of his observations of stars of type O5 and earlier and shows that inclusion of the line blocking between 228 and 912 A solves the problem found by Herrero et al. (1992) in the determination of their stellar parameters. The author studies the influence of the line blocking and other effects on the mass and helium discrepancies and shows that the first one is reduced by the use of spherical, non hydrostatic model atmospheres and that the second one is probably due to exposure of CNO material.