Abstract: In recent years, the observational evidence showing that single evolutionary models cannot fully explain the properties of massive stars has significantly increased. Despite the efforts to improve physical prescriptions like rotational mixing, massive stars’ evolution remains an open question. In this seminar, I will discuss the helium and nitrogen surface abundances in a sample of O Main Sequence stars. In particular, I will focus on the discrepancy between predictions from the CNO cycle and the observed abundances for ∼80% of He-enriched stars (He/H≥0.12), which suggests that binary interactions play a crucial role in the He enrichment observed in these objects during their Main Sequence phase. Complementary observational information, along with cutting-edge binary population synthesis, support that past mass transfer events in massive binary systems are likely the primary origin of this subset of stars.