Charles would have nothing to fear power-wise, he was not a big puncher with 5-6oz gloves much less with 10oz gloves. Charles was not a big puncher but had speed and skill. IMO Charles would cop a decision.
Overlin and Charles fought twice. A loss and a draw. In the first fight Charles was 17-0, the second fight Charles was 21-1. Overlin was vastly more experienced having 150 pro fights(122-19-6) in the first fight and 130-19-7 for the second. This was Charles' first big time opponent. Let's make it more equal by the time Charles had his 50th fight it would've been no contest with Overlin.
Charles by easy UD. Greb would be slowed down a bit and his arm punches would be ineffective against the Cincinnati Cobra.