The presence of a nearby companion significantly alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, X-ray binaries, and gamma-ray bursts. This study simultaneously measures all relevant binary characteristics in a sample of Galactic massive O stars and quantifies the frequency and nature of binary interactions. Over seventy percent of all massive stars will exchange mass with a companion, resulting in a binary merger in one-third of cases. These findings suggest that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae. The study analyzes the O star population of six nearby Galactic open stellar clusters, using spectroscopy to measure orbital periods and mass ratios. The results indicate that 71% of all stars born as O-type interact with a companion, with over half doing so before leaving the main sequence. This highlights the importance of considering binary interactions in understanding the formation and evolution of massive stars and distant star-forming galaxies.The presence of a nearby companion significantly alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, X-ray binaries, and gamma-ray bursts. This study simultaneously measures all relevant binary characteristics in a sample of Galactic massive O stars and quantifies the frequency and nature of binary interactions. Over seventy percent of all massive stars will exchange mass with a companion, resulting in a binary merger in one-third of cases. These findings suggest that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae. The study analyzes the O star population of six nearby Galactic open stellar clusters, using spectroscopy to measure orbital periods and mass ratios. The results indicate that 71% of all stars born as O-type interact with a companion, with over half doing so before leaving the main sequence. This highlights the importance of considering binary interactions in understanding the formation and evolution of massive stars and distant star-forming galaxies.