This is a no-brainer.
Lesnar is a former NCAA Division I Wrestling Champion, amassing a phenomenal record of 106-5 during his collegiate years of competition.
Alistair Overeem is great off his back and knows his way around submissions. However, he has never been tested against a great wrestler and, at UFC 141, he has been thrown into the deep end against a wrestler like Lesnar.
Combined with his strength and power, Lesnar can use his wrestling skills to keep Overeem down and keep him in control, both on the ground and in the clinch.
Furthermore, Lesnar has shown that he actually has tremendous cardio strength. I mean this in the sense that, yes, Lesnar does get tired and, yes, he does slow down as the fight progresses into the later rounds—but he does not gas out completely. In fact, it is his cardio that has saved him from Randy Couture and Shane Carwin.
Alistair Overeem has only had five fights go to a decision. Is that a testament to his ability to finish his opponents? Maybe. But then you have to consider the opponents that he has defeated, linking back to the first point I made: He has been finished by the majority of the "big-names" he has faced in the very first round.
In his fight against Fabricio Werdum, Overeem slowed down and was as sloppy as Werdum as the rounds progressed. Overeem's cardio has always been in doubt and he will struggle if the fight gets taken into the later rounds, especially with Lesnar lying on top of him.
Lesnar is great from the top position. If he can get Overeem to the ground, he can deliver ground and pound to neutralize Overeem's submissions skills, echoing what he did against Randy Couture, which ultimately proved very successful for Lesnar.