The most recent results of the LIGO-Virgo collaboration urge us to explain the formation of black holes with mass ~50-130 Msun (hereafter, oversize black holes), which was previously considered a forbidden range for stellar-born compact objects. Star cluster dynamics offers the perfect environment to build such oversize black holes. In my talk, I will discuss the impact of stellar collisions on the formation of (very)massive stars and oversize black holes, by means of new hydro-dynamical simulations, entropy-sorting algorithms and stellar evolution calculations. The density and mass of the parent star cluster are two key features to understand the demography of (oversize) binary black holes, as we probed with direct N-body simulations. Moving to a larger scale, I will speculate about the host galaxy candidates of binary black holes versus other binary compact objects. Binary black holes typically form in metal-poor dwarf galaxies, while binary neutron stars are almost insensitive to the metallicity of their host galaxy. In both cases, we find a strong correlation with the star-formation rate and the mass of the host galaxy. This prediction is crucial to guide the search for optical counterparts to binary compact object mergers.