Overeem is a superior fighter to Roy in basically every way except ability to take punishment. That's why Roy is the underdog. I think Overeem will look to work in the clinch and take Roy down for some ground and pound. If he can avoid Nelson's power punches, especially the overhand right, then it should be an easy win for him.
That said Overeem always seems to find ways to lose fights against technically inferior opponents that he's dominating. At times he's shown poor fight IQ and he doesn't respond well to taking shots. It's a shame for him that this is the case. His striking is fantastic and he has a vastly underrated grappling game. If he had a solid chin like say Dos Santos, Nelson, Miocic...etc then he would easily be a top heavyweight, possibly even champion.
I'm picking Overeem but I won't be at all surprised if he gets caught and KO'd/TKO'd.