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Massive stars play a key role in the evolution of the Universe. Our goal isto compare observed and predicted properties of single Galactic O stars toidentify and constrain uncertain physical parameters and processes in stellarevolution and atmosphere models. We used a sample of 53 objects with spectraltypes from O3 to O9.7. For 30 of these, we determined the main photospheric andwind parameters, using optical spectroscopy and applying the FASTWIND code. Forthe remaining objects, literature data, obtained by means of the CMFGEN code,were used instead. The properties of our sample were compared to publishedpredictions based on two grids evolution models that include rotationallyinduced mixing. Within each luminosity class, we find a close correlation of Nsurface abundance and luminosity, and a stronger N enrichment in more massiveand evolved O stars. Additionally, a correlation of the surface nitrogen andhelium abundances is observed. The large number of nitrogen-enriched starsabove ~30 solar masses argues for rotationally induced mixing as the mostlikely explanation. However, none of the considered models can match theobserved trends correctly, especially in the high mass regime. We confirm massdiscrepancy for objects in the low mass O-star regime. We conclude that therotationally induced mixing of helium to the stellar surface is too strong insome of the models. We also suggest that present inadequacies of the models torepresent the N enrichment in more massive stars with relatively slow rotationmight be related to problematic efficiencies of rotational mixing. We are leftwith a picture in which invoking binarity and magnetic fields is required toachieve a more complete agreement of the observed surface properties of apopulation of massive main- sequence stars with corresponding evolutionarymodels.

Spectroscopic and physical parameters of Galactic O-type stars. III. Mass discrepancy and rotational mixing