Binary interaction dominates the evolution of massive stars. A study of 71 O-type stars in six Galactic open clusters reveals that over 69% of these stars are part of binary systems. The research shows that more than half of these stars interact with a companion before leaving the main sequence, leading to binary mergers in one-third of cases. These findings challenge previous estimates and indicate that binary interactions are the dominant factor in the evolution of massive stars. Massive stars, with masses over 15 times that of the Sun, are rare and short-lived, but they play a crucial role in enriching the interstellar medium and driving galaxy chemical evolution. In binary systems, the presence of a companion significantly alters the evolutionary path of massive stars. Close binaries can lead to mass transfer, angular momentum exchange, and even mergers. The nature of these interactions depends on the initial orbital period and mass ratio. The study used spectroscopy to measure orbital periods and mass ratios, revealing that the intrinsic binary fraction is 0.69 ± 0.09. The observed distributions of orbital periods and mass ratios show that binary interactions are common and have a significant impact on the evolution of massive stars. The results suggest that a large fraction of massive stars are affected by binary interactions before they explode as supernovae. The study also found that 33% of O stars are stripped of their envelopes before exploding as hydrogen-deficient supernovae. This aligns with the observed fraction of hydrogen-poor supernovae. The findings imply that binary interactions are a major source of hydrogen-poor supernovae and that more than half of the progenitors of hydrogen-rich supernovae are merged stars or binary mass gainers. The results challenge current star formation theories and suggest that binary interactions are a key factor in the formation and evolution of massive stars. The study highlights the importance of binary interactions in shaping the properties of massive stars and their supernovae, and in understanding the formation of distant star-forming galaxies.Binary interaction dominates the evolution of massive stars. A study of 71 O-type stars in six Galactic open clusters reveals that over 69% of these stars are part of binary systems. The research shows that more than half of these stars interact with a companion before leaving the main sequence, leading to binary mergers in one-third of cases. These findings challenge previous estimates and indicate that binary interactions are the dominant factor in the evolution of massive stars. Massive stars, with masses over 15 times that of the Sun, are rare and short-lived, but they play a crucial role in enriching the interstellar medium and driving galaxy chemical evolution. In binary systems, the presence of a companion significantly alters the evolutionary path of massive stars. Close binaries can lead to mass transfer, angular momentum exchange, and even mergers. The nature of these interactions depends on the initial orbital period and mass ratio. The study used spectroscopy to measure orbital periods and mass ratios, revealing that the intrinsic binary fraction is 0.69 ± 0.09. The observed distributions of orbital periods and mass ratios show that binary interactions are common and have a significant impact on the evolution of massive stars. The results suggest that a large fraction of massive stars are affected by binary interactions before they explode as supernovae. The study also found that 33% of O stars are stripped of their envelopes before exploding as hydrogen-deficient supernovae. This aligns with the observed fraction of hydrogen-poor supernovae. The findings imply that binary interactions are a major source of hydrogen-poor supernovae and that more than half of the progenitors of hydrogen-rich supernovae are merged stars or binary mass gainers. The results challenge current star formation theories and suggest that binary interactions are a key factor in the formation and evolution of massive stars. The study highlights the importance of binary interactions in shaping the properties of massive stars and their supernovae, and in understanding the formation of distant star-forming galaxies.