For decades, the origin of helium enrichment in O-type stars has remained an open question. We investigate the correlation between surface helium abundance ( ) and orbital parameters for a sample of 45 O-type SB1 systems, including Cyg X-1. We find seven He-rich systems, all of which are concentrated at short orbital periods (P!łesssim!15 days) and are classified as RW. In addition, four of them present relatively high eccentricities, while the other three have ellipsoidal variations. We argue that these properties are the result of binary interaction. These findings provide strong observational evidence that binary interaction is the dominant origin of helium enrichment in O-type stars. This result has important implications for the treatment of chemical mixing and surface abundances in massive-star evolutionary models, and it establishes helium enrichment as a promising observational tracer for identifying post-interaction binary systems.