There is increasing evidence that single-star evolutionary models are unable to reproduce all of the observational properties of massive stars. Binary interaction has emerged as a key factor in the evolution of a significant fraction of massive stars. In this study, we investigate the helium (YHe) and nitrogen (ϵN) surface abundances in a comprehensive sample of 180 Galactic O-type stars with projected rotational velocities of ≤150 km s‑1. We found a subsample (∼20% of the total, and ∼80% of the stars with YHe ≥ 0.12) with a YHe and ϵN combined pattern that is unexplainable by single-star evolution. We argue that the stars with anomalous surface abundance patterns are binary interaction products.