Context. The presence of massive O-type stars with surfaces enriched by CNO-cycle products has been known since the early 1980s. For many years, internal rotational mixing was assumed to be the dominant mechanism responsible for this chemical contamination. However, accumulating evidence suggests that binary interaction may play an equally important, if not dominant, role.

Aims. Our aim was to carry out a large-scale investigation of surface helium (He) abundances in Galactic O-type stars, based on the results from the analysis of high-quality spectroscopic data from the IACOB project.

Methods. We performed a homogeneous spectroscopic analysis of 318 Galactic O-type stars with the iacob-broad and fastwind/iacob-gbat tools, deriving rotational velocities, atmospheric parameters, and He abundances. We also accounted for the influence of binarity and parameter degeneracies on the abundance determinations.

Results. We present homogeneously determined surface He abundances (YHe = NHe/NH) for the largest, statistically significant sample to date of Galactic O-type stars. About 60% of the stars show He abundances consistent with the cosmic abundance standard of YHe = 0.098 ± 0.002. For another 18% of the stars, we obtain anomalously low He abundance estimates, reaching values down to 0.07. These unusual He abundances might be a consequence of flux contamination of the analysed spectra by a faint companion. The remaining 22% display clear He enrichment (YHe 0.13). We provide observational evidence indicating that most of these He-enriched stars are likely the products of binary interaction.

Conclusions. Our study highlights how large spectroscopic surveys are gradually opening robust observational avenues to identify the products of massive binary interaction. It also emphasises the need for caution when interpreting the spectroscopic properties of apparently single O-type stars. A significant fraction may in fact be the outcome of binary evolution rather than isolated stellar birth.