Robert takes this.
A lot of people would dispute with me about that, as the Mountain fell before Oberyn, but here’s the thing: the Mountain was a brute with a sword. He had height and strength, but not much else. He was physically imposing but not particularly intelligent. As a swordsman, he was decent at best, but his size and power were really all he had going for him. Otherwise, he might as well have been a regular soldier.
Robert, on the other hand, was a hulking monstrosity, ridiculously skilled with his warhammer. He was like the mortal version of Thor, a Viking berserker. The more you hit him, the angrier he got. Poison wouldn't even matter—unless it acted instantly and killed him on the spot, he'd ignore it until after he'd left his foe a broken wreck.
Simply put, the Mountain had some skill with a sword, but Robert Baratheon had an incredible mastery of his warhammer. And Robert would have completely overpowered Oberyn.
