In fight scenes, the fighter usually defeats a swarm of opponents without getting hit or tired. The movie Old Boy didn't have that and in my opinion, was the most realistic fight scene ever. My question is how realistic was it to defeat multiple opponents in the movie.
That it could happen is plausible as seen by the existence of real-life examples of one person defeating a mob. In general, this becomes less common without armor or weapons because all it takes is one lucky hit to create enough of an opening for others to swarm and pigpile you. Oh Dae-su has the benefit of a weapon. Weapons draw attention and make each individual a little more afraid to attack. He's also more than a little bit crazy, and each individual is likely to take that into account when they consider whether they, individually, will escape confronting him with life and limb intact, plus he's likely not feeling any pain until later. Add to that that the thugs probably are not trained fighters and are used to beating people up who don't fight back. Take all of this together, and you get a plausible situation for one man to survive the fight, albeit far from unscathed. If a handful of them had rushed him and pinned him down where he couldn't get leverage, they would have defeated him, but they were all seeking to minimize their individual risk and therefore they were defeated as a whole.