Rotation is one of the important parameters affecting the evolution and final fate of massive stars. Still the origin of fast- rotating stars in the massive OB domain remains unclear. Although the high spin rates detected in about 20-30% of these stars could just be an imprint of the star formation process, alternative scenarios -supported by the fact that the majority of massive stars are born in binary or multiple systems- propose that most of them are the result of binary interaction. The understanding of binary evolution in massive stars, as well as the impact that it can have on the stellar spin of the two individual components as the system evolve, has been a hot topic in the last decade. In this work, we aim at investigating the binary status of a statistically meaningful sample of Galactic fast-rotating O stars and B supergiants by performing a comprehensive multi-epoch radial velocity analysis of new high quality spectroscopic observation gathered in the framework of the IACOB project. Our ultimate objective is to provide solid empirical constraints to be considered by any theoretical attempt to investigate the impact and characteristics of binary evolution in the massive star domain.