Actually he'd be pretty safe if he can get the clinch against the cage and keep Overeem there. When a fighter is pressed up on the cage, he can't get much power behind any of his strikes since his feet are level and there's no way to get a good weight transfer, step-in, or hip rotation into the strike. The danger comes if/when Overeem manages to get his back off the cage and start turning into a position where he drive off his feet or get some hip rotation into his knees so he can put serious power into the strikes. Worst case for Cain would be if reverses position on him against the fence, in which case he's going to eat full power knees and punches till he drops.
In the clinch against the cage, Cain will either need to GTFO fast or grab a leg for a takedown as soon as he feels like he's losing his position or control. If he gets a good hold of a leg he can't be kneed and if he completes the takedown then Overeem will be in for a beating.
Which is pretty probable with the way Overeem tosses around people of the size of Lesnar by pure strength.
I don't think Reem will throw many kicks or knees, he'll be too worried about the TD.
He wasn't too worried about Lesnar's TDs.
Velasquez key against Overeem is his cardio. Overeem has only a round or two, then his muscle mass will have burned all the energy in his body. If Overeem doesn't finish Velasquez in the first round, he is in bigger trouble than Dos Santos was, but he has a better chance to finish it in the first, because unlike Dos Santos Overeem punishes TD attempts.