The main character is going around murdering people, in the hallway and the elevator but no police ever finds him.
In the final act of the film , There's been a murder on a floor in the building, and one person shot himself in the elevator, Where is the police? How did Dae su get away from that place being in a condition he was? Was there seriously no one in the building at that time apart from the henchmen of the villian.
Another minor thing that I wanna ask is The main tagline of the film is
"Laugh and the world laughs with you; weep, and you weep alone"
It kinda hints that Dae-su was laughing about them and making fun of the villain and his sister but as far as I remember he didn't.
He only told it to one person and it didn't seem like he was laughing at them or mocking them at all. It really hurts the film's message I think because here the person who was punished for 15 years is not the person who was laughing at the them and spreading this rumour. Shouldn't the villian with all his money be going after other kids who were spreading that rumor more ?
Only these minor problems i had with Oldboy which i otherwise loved