Brock is bigger, stronger, faster, and a more skilled fighter than Orton and he knows that. But, Orton just needs one opening and he change the momentum of the match. The RKO is one of the fastest finishers in the WWE and Orton can be hit it from anywhere in the ring.
The biggest threat to Orton is if Lesnar gets a good solid hold on him. Lesnar is a trained MMA fighter who uses that skill in a bulk of his move set. From his Suplex City combo to his finisher the Kimura lock, Lesnar built his fighting style around moves that will wear down opponents or finish them quickly.
Orton doesn’t have any known submission moves and his style clashes with Lesnar’s, but not in a way that would give him the upper hand. If anything it would hinder Orton from starting off with a realistic. This is why I believe Lesnar will win the match convincingly.