Stellar rotation is of key importance for the formation process, evolution, and final fate of massive stars. Performing a reassessment of the empirical rotational properties of Galactic massive O-type stars using the results from a detailed analysis of ground-based multi-epoch optical spectra obtained in the framework of the IACOB & OWN surveys. Using high-quality optical spectroscopy we establish the velocity distribution for a sample of 285 apparently single and single line spectroscopic binary (SB1) Galactic O-type stars. We also make use of the rest of the parameters from the quantitative spectroscopic analysis presented in prior IACOB papers (mainly Teff, gravity, and multiplicity) to study the vsini behavior and evolution from the comparison of subsamples in different regions of the spectroscopic HR diagram. Our results are compared to the main predictions -- regarding current and initial rotational velocities -- of two sets of well-established evolutionary models for single stars, as well as from population synthesis simulations of massive stars including binary interaction. We reassess the known bimodal nature of the vsini distribution, and find a non negligible difference between the vsini distribution of single and SB1 stars. We provide empirical evidence supporting the proposed scenario that the tail of fast rotators is mainly produced by binary interactions. Stars with extreme rotation (>300km/s) appear as single stars and located in the lower zone of the sHR. We notice little rotational braking during the main sequence, in addition to being independent of mass (and wind strength). The rotation rates of the youngest observed stars lean to an empirical initial velocity distribution with <20% of critical velocity. Lastly, there seems to persist a limit in vsini detection below 40-50km/s, especially in the upper part of the sHRD, possibly associated with the effect of microturbulence in the measurement methodology used.