The name of the throw will give you a big hint of how it works, the translation of Osoto makikomi is “big outer wraparound”. “maki” means “wrapping” and “komi” means “applying around”.
So basically, the throw consists of taking control of the opponent’s arm, trapping it, then spinning around as if you want to wrap a sheet around you. When you take the arm and do the spinning, the opponent’s body follows their trapped arm, and that’s how they fall on the ground.