  Stellar rotation is of key importance for the formation process, evolution,
and final fate of massive stars. In this paper we review results from the study
of the spin rate properties of a sample of more than 400 Galactic O-type stars
surveyed by the IACOB and OWN projects. By combining vsini, Teff, and logg
estimates (resulting from a detailed quantitative spectroscopic analysis) with
information about the spectroscopic binarity status for an important fraction
of the stars in the sample, we provide a renewed overview about how the
empirical distribution of projected rotational velocities in the O-star domain
depends on mass, evolutionary and binary status. The obtained distributions are
then compared with predictions of several state-of-the-art evolutionary models
for single stars, as well as from population synthesis simulations including
binary interaction, and used to provide hints about the initial velocity
distribution of stars with masses in the range ~15-80 Msol.
