Reem has all the tools to win. He could keep range & out strike Nelson, ala Miocic, or he could look to control Nelson on the ground with his insane strength & possibly get a ground & pound KO. Of course, he has all the skills to land a KO on the feet as well, with pretty much any of his weapons, especially in the clinch. Truthfully, he's more talented in nearly ever area of the game. I could even see Overeem snagging a guillotine or an americana if he sees an opening.
That said, I will never feel safe picking Overeem to win a fight. Nelson is tough as hell & if it comes down to taking an uberknee to the jaw to land a glancing punch, Nelson will do it, and Reem will fall. I've got Nelson in this one.