Distribution of the ratio of the equatorial surface rotation velocity to critical rotation velocity for the OB stars in OB+BH binaries at the moment of BH formation, as predicted by our population synthesis model (top panel). Bottom panel: corresponding distribution of the absolute equatorial surface rotation velocities of the OB stars as obtained in the indicated mass bins. In both plots, the small peak near zero rotation is due to the widest, non-interacting binaries; it is non-physical and should be disregarded.