Here is how the fight will go:
Brock will bull rush towards Overeem and will be successful in taking him down. Brock is a monster in the first round when he is fresh and he can take anyone down. Imagine having a 300 lbs monster rushing at you at 25mp/h. Also, Overeem is a tall, lanky guy at 6'5, and the taller you are the easier you fall...
Brock takes him down and is able to hold Overeem down for most of the round but doesen't do any damage due to Overeem's strength which keeps Brock clinched and thus unable to ground-and-pound successfully.
At round 2, Brock is more tired but tries to take Overeem down again. Overeem hits Brock with several straight jabs to the face as Brock comes in, and as Brock becomes disoriented, Overeem clinches and hits Brock with several knees to the head. Brock tries to take him down again, but Overeem sprawls and clinches with Brock again, hitting him with more knees to the head. As Brock presses forward, Overeem keeps sprawling Brock's takedown attempts, clinching and using knees and elbows. Brock survives round two, but is a mess.
At round 3, Brock no longer has any wind left to bull rush, so he makes inept attempts at taking Overeem down. Overeem keeps his distance by kicking him in the legs and ribs. Into the round, Brock is exhausted and his strength is gone. Overeem moves in with jabs and Brock collapses onto the ground. Overeem hits him a few more times, the referee realizes Brock can't do anything about it and interrupts the fight. Overeem TKOs Brock.
My prediction: Overeem wins by 2nd or 3rd round TKO.
Whether it ends in the second or third round depends on whether Brock can still bull rush Overeem at the beggining of round 2. Probably not. If he can, then he takes the fight to round 3 but then he loses for sure.
SUCKMYMUSCLE