The save seems to have 3 things: The doctor's "memory confusion" hypothesis was always wrong. Ohma had trauma that solidly and falsely limited his power, until he faced it in this last chapter. Second, with this trauma resolution, Ohma's already impressive willpower taps into his latent strength that dominated the presently superior Kiryu Setsuna--or at least that's how I see Setsuna. Third, what the hell is up with this series and people fighting on despite horrible damage? I can't explain away the doctor's diagnosis, nor can I explain Ohma's overburdened heart, nor how he doesn't have terrible internal bleeding from all the broken bones which likely haven't healed.
Then again, the whole "skull helmet"from being often hit with a sledgehammer is wildly implausible, as well as Haru/Haruo losing weight in a medically impossible means, both show that this series takes artistic license to human physiology for the mechanics of the fighters as well as for the theme of the series.
Ohma's perseverance fits both categories, so I'll let it slide.
@Nintendude1997, agreed, but it'll be both a contest of strength and technique.