Overeem- Must come out fast early and throw every trick he has at Ngannou, with a lot of variety. He must finish Ngannou early or Ngannou will Time him and knock him out.
Ngannou- Must be patient. If he comes out aggressive early, Overeem can take him down or counter him. Ngannou should try and counter Overeem, but not give him space to land kicks. Ngannou' reach should come into play, as he can keep Overeem on his toes with his reach and then Time him as he comes in.
Overall, if the two fighters are afraid to engage each other, this match could be a sleeper or a barn burner. I believe that if Overeem is to win, it will be early. If Ngannou does what Werdum does and sits back and counters, the shot he lands will knock Overeem out.
I think 45%- Ngannou by KO/Sub
35%- Overeem by KO/ Sub
12%- Overeem by Dec
8%- Ngannou by Dec