Overeem wins.
If jr thought Velasquez hit heavy, Overeem will learn him. Overeem can take shots, and will keep the reach advantage with superior kicks. I predict both men will trade some amazing shots though.
Overeem and JDS are both masters at takedown defense, but neither one is truly great at takedowns. Both will probably be content to stand and strike. Either man can knock the other out, but I don't see JDS getting inside enough to get a clean shot at Overeem. AO will keep his distance with low kicks.