If you loved Oldboy, try Sympathy for Mr. Vengeance
A bridge between a film you've already seen and one most people haven't. Sympathy for Mr. Vengeance has roughly 5.8× fewer votes than Oldboy — it's a deeper cut, not a mainstream recommendation. Here's what they share, and what the second one does that the first one doesn't.
What they share
Both films are directed by Park Chan-wook, and they sit in Action / Drama / Thriller territory. If that's the register that drew you to Oldboy, the second film will land in a comparable space — through a different lens.
What Sympathy for Mr. Vengeance is
Seoul, rainy night, a payphone rings. A desperate plan forms, a kidnapping unfolds, things spiral. Park Chan-wook lays the groundwork for his vengeance trilogy.

