Unable to deal with a reality where hypnotic trickery and manipulation has forced him into a carnal relationship with his own daughter, Oldboy protagonist Dae-su cuts out his own tongue and turns to hypnosis to strike these events from his memory.
The final scene sees Dae-su reunited with his daughter, but unease grips him and the viewer alike as his smile gradually fades, giving way to a look of anguish.
Director Park Chan-Wook's intention wasn't to inspire the viewer into detective work in search of definitive answers to the multiple questions this ending leaves behind. The purpose of the conclusion was to leave room for assumption and speculation.
Nobody knows whether the hypnosis was effective, whether it even took place, or how much time had elapsed by this point. Beyond Dae-su's expression, there are no clues to suggest any of the theories that have sprung up around these questions are right or wrong, but the fact they sprung up is no doubt what Chan-Wook wanted.
The US Oldboy remake released in 2013 also gave rise to a burning question: why the hell was this even allowed to happen?