KO has a very difficult toolset to defend. As a 7 footer, they have to put a big out to defend him. Because he shoots the 3 with great accuracy, this forces the big to come out to 3 point territory to defend, where they are less comfortable, and also pulls the big away from the paint defensively.
On top of that Kelly has drive and dish skills, he can either drive on the likely slower big, or when he attracts attention, pass to the open man. When you add it all up, you have potential for a nightmare matchup. Kelly actually cashed in those tools against the 76ers for 30 pts on 12 for 17 shooting with three 3 pointers.
I feel it is just a matter of time and confidence for Kelly to put it all together. And luckily for him his offensive game is not predicated on blowing by players or outjumping them. And there are not a lot of super athletic defensive 4/5s who can guard him out on the perimeter.