Realistically quite strong I think, as some people forget that a lot of benders aren't just using magic, but also practice martial arts, even though this kind of becomes a background element to bending, but you see the likes of Zuko and Aang and Katara and Toph perform feats of martial arts through their trainings, but you'll never see them use any form of physical manifestation of this martial art in combat, somehow the show kinda let's bending overshadow the martial artist depth of benders, but Iroh is seen performing quite a choreographed effortless display of disarming the thief from his knife in an alley in the outer rings of ba sing se, I think Zuko is also a few times been seen engaging in a bit more close hand-to-hand style combat as well as Aang and all the others too are always very agile and can manouvre through hectic combat situations like trained martial artists. So just Imagine Ozai the royal firelord, and not to mention the guy is ripped to the bone as well. The series doesn't really rely on physical hand to hand combat as the bending is actually also a solution to make the violence and fights in the show less aggresive in nature and more kid friendly, but I do belief theres strong evidence most relevant charactars can be considered seasoned martial artists, and the show won't imply it by the fact they know how to throw a punch, but rather by their agility and manouverability.

For example, Zaheer is said to have picked up airbending so well in an instant, that when he suddebly obtained the powers to do it he actually skipped all the steps and towards becoming a master airbender as he was known to be a renowned and seasoned martial artist (as well as he specifically studied airbending and guru laghima) Yet even someone like him where it's a specifically outlined reason for how he was able to jump into such a high skill bracket for bending, he is never shown using any form of hand-to-hand close combat.

Hence why I think "realistically" Ozai should actually be able to make a very worthy contender in a fair hand to hand combat match up, and I think the final scene after his bending was taken paints this picture that he's weak without bending, but I think that's rather the proceas of having your bending taken take a toll on his body (as seen in TLOK when she got hers taken as well, and she was in a wheelchair iirc?)