The MONOS project has obtained WEAVE LIFU observations to study the cores of nine stellar clusters containing O and B stars. Most O stars form in multiple systems, which allows for the determination of stellar masses through their orbits. However, this process is complicated by the wide range of orbital periods and the presence of complex systems like triples and higher-order multiples. The initial analysis focuses on assessing the quality of the WEAVE LIFU spectra and providing spectral classifications. Future work aims to determine spectroscopic binarity and study stellar orbits using radial velocity and photometry from TESS and Gaia.