Sophocles did it earlier, and better.

Rating: 7/10 (25 August 2005)

There seems to be a great deal of backlash to this film, mostly resting on its supposed technical ineptness. I've heard poor acting - not true. Plot holes? Not particularly, if you take the film on its own terms as a world where hypnotism can actually do such things. Too much violence and gore? I've seen gorier and more disturbing films which did not produce quite the distaste and bitterness in my mouth that Oldboy managed.
No, I suggest that in order to understand this movie, you have to understand some fundamentals of drama, and in order to understand why it feels so emotionally unrewarding one has to turn to Aristotle's poetics. Start with books ten and eleven, if you want to get right to the meat (but you'll be doing yourself a disservice). It details the dynamics of reversal and recognition, both of which almost overwhelm the ending.
The film follows much of Aristotle's demands for making tragedy excite pity and fear. We have the "man who is not eminently good and just, yet whose misfortune is brought about not by vice or depravity, but by some error or frailty" and as said before recognition. And yet, while watching the film I felt little pity and no fear. Well, the teeth-torture had me cringing, but that's a shock-horror trope which cannot help but have that effect. I think because the film may actually be to wrapped up in theory, caught up in so much crude narrative mechanics that the motivations of the characters are not properly fleshed out. This is a very fatalistic universe which Chan-wook Park has created. The director has embraced the modern shock aesthetic too firmly, and mostly abandoned the poetry which a tragedy requires. What poetry there is lies in some of the films cinematography, when it is not too caught up with spectacle and forcibly impressing the viewer.
This is technically a good film, but seems to me to lack grace. The ambiguous ending also seems half-baked. We understand that this character has gone from a good situation to a bad situation, but does the last-minute oddness make it slightly less bad? Perhaps the only way to deal with such a traumatic situation? What was the intent, exactly, to have the relationship between Oldboy and his lover to continue as it does? I don't think that this is a polemic in favor of decriminalizing incest - I suspect that the director just thought it would be another cool way to mirror Oedipus Rex, where the father, blinded, walks off into the sunset with his daughter, to wander the world.
Hopefully, the next film will be more Sappho-influenced. For a better, more tender example of modern reversal and recognition, look no further than eternal sunshine of the spotless mind.