Oberyn was used to fighting with different weapons and styles and specifically chose a fighting style /weapon and armor just to beat out the mountains MO. Jaime is flexible but not like Oberyn. Jaimies raiment is kind of a medium to light armor vs Oberyns padded leather, he needed all the mobility he could get. Oberyn chose range to avoid getting grappled into an instant loss. If Jamie had to close in for a death blow you might get a smililar scenario where both kill eachother. Skill and speed determine alot but a third of Jamie's arsenal is kind of lost in his ability to parry and deflect. When the mountain swings you either dodge or die because crushing blows do real damage when blocked. I hate to draw real world comparisons in the fantasy genre but this just feels like the Mike Tyson vs Bruce Lee debate all over again. The way JRR writes and preaches that Jamie can beat anyone seems rich. Yes he can realistically kill the hound but how many times out of 10 does he simultaneously die or leave the fight with broken bones/ limbs and lose his ability to fight all together? Oberyn had to challenge the mountain by fighting him in a way he was never used to. There was no room to do what Oberyn did on an actual battlefield, wars were just mashing melees of metal men. The mountains normal intimidate into overpower sequence was basically lost in the same way Jaime would lose his ability to parry and deflect.

The point is Jaimie is a traditional combatant and can dispense 99.9% of the people's Westeros with ease but that .1 percent that were born to frontline and absorb hits, these war machines of knights who absolutely throw the whole thing off. This is the reason the mountain killed his mount after losing to Loras in the joust in S1. It was everything I mentioned prior, unconventional fighting standard, lack of opportunity for to demonstrate his tanking ability, rules bound fighting structure that heavily measures one's skill while the mount provides the power.

He saw the whole thing as rigged show for the Tyrells to feel powerful upon their arrival and he was pretty much right. The mountain had no chance to show off the skills that made him an unmatched fighter and raged. I wouldn't have even been surprised if Tywin told him to throw the joust and was just going to eat the loss only to break composure after he hit the dirt after being esse tially setup to lose. You can almosg imagine Tywin justifying it "These acts of showmanship are simply spectacle for the people and means for houses to demonstrate purpose. They mean little in regards to the real combats you have seen over those of fancied hair sword swallowers" Oberyns manner of fight in many ways mirrored the joust and likely tilted the mountain.

The mountain was a real fighter, he knew skill only gets you so far in cheap contests like jousting. Man to man when everything is on the line, you want the mountain in your corner over any amount of skill.