Was the revenge justified?

I don't find any plot dilemma here at all. Oh Dae Su was imprisoned because the antagonist wanted him to suffer. Whether he deserved this is never discussed because the punishment came from a sociopath with the same anger and drive for revenge from when he was a kid.

He believed (rightfully so) that Oh Dae Su was the cause for his sister's death as they could never survive in a strict Korean society knowing about this incestuous pregnancy.

The antagonist lived a life loving his sister and also plotting his revenge. Over the years this drive for revenge would only grow without closure. This would be the cause for plotting and scheming on a sociopathic level.

After 15 or so years he finally decided Oh Dea Su was ready to feel what it was to love one from your own family. Here begins the elaborate plan for Oh Dea Su to unknowingly build an intimate (and sexual) realationship with his own daughter.

When the antagonist finallly lets Oh Dea Su know the whole story he's finally acomplished what he wanted, for Oh Dea Su to feel the same feelings he did. With that done he now feels he has no other purpose in life because he spent so long plotting his revenge, and so kills himself.

Oh Dea Su lives the rest of his life with his daughter all the while knowing the truth in the back of his mind. And there is where our antagonist wins. He wanted Oh Dea Su to feel the shame of a strongly incestuous relationship.

I hope this answers your question. With these long answers it's easy to sidetrack.

Well that is the whole point of the movie. The antagonist wanted to make him what he felt once. He wanted Oh Dae su to live life knowing of his incest. It makes no difference whether this is intentional or not. Woo-jin summed this up at the very end of the movie when he said something like

If I can live my life loving my sister you can do to.

He didn't want any revenge of any sort. He just wanted to make the situation of Oh dae su like his own. Remember he just wanted to make the environment of Oh dae su to be such that he would fall in love with his own daughter unknowingly, so he might feel what he once felt.

That for me is the whole point of the movie.

The answer to this question may vary from person to person because it depends on one's personal opinion. According to me though, I agree tht the revenge was unjustified. Oh Dae-su did not deserve the treatment he got. What was his crime ? He saw something and he told someone about it. Now there is one points that I would like to highlight -

It wasn't like Oh Dae-su spread a lie. He told what he actually saw, what actually happened and which wasn't a normal thing. Isn't it human tendency to talk to someone if one saw or witnessed or experienced something highly unusual or bizzare ?

What happened to the antagonist's sister, Oh Dae-su was not responsible for it. It was bound to happen. Someone must have noticed it sooner or later.

If you watch the 2006 hindi movie 'Zinda', which is largely based on Old Boy, there are small changes from the original plot. In that movie the revnge is justified because Oh Dae-su's character spreads a false rumor which leads to the girl's death.

However in the original movie, the revenge is unjustified. That's what I believe.

The story doesn't suggest that the revenge is justified. You rightly judge the villain's character as a sociopath, which he is clearly depicted in the movie, then you shouldn't expect that somehow the villain's revenge against the innocent Dae-Su is to be seen justified. The movie doesn't show Dae-Su as the villain that you assume we are to sympathize with the antagonist. The only sympathetic suggestion towards him was showing he did not murder his sister, but nothing more than that.

Oh Dae Su never knew who Soo Ah was, she was just a girl he was showing off to, and happened to catch getting up to mischief. He had no idea that they were brother and sister - so how can he be to blame. His friend Joo Hwan spread the story after Dae Su left so surely it was his punishment to bear? After all he identified the girl!

I'd like to know if the hypnosis was successful because that last shot of Dae Su did not look like a man free and in love.

You are not grasping the central theme. What the antagonist wants is for society to understand that he and her sister did nothing wrong, but incapable of that, he settled for Oh Dae-su. This is revealed by the scene where he stabs Oh's friend for perpetuating the rumors that his sister is a slut, Oh's friend represents society, anyway...

Everything the antagonist did, Oh Dae-su ends up doing - he took revenge on people who, though not entirely innocent, weren't actually responsible for his imprisonment, and, in the end he intentionally chooses incest by going to the hypnotist - because both characters are so human. You think the antogonist is a sociopath, sure, you are entirely like Oh Dae-su before he was put forcibly into the antagonist's shoes, then it is revealed that Oh Dae-su would rather take blissful ignorance than give up his love. (This ending is the real revenge, more on that later) He shows us that love is not a choice, but more generally, the evil you fight is more like you than you think, for "when you look into the abyss, the abyss also looks into you" - Nietzsche

We are meant to sympathize with Oh Dae-su, and then gradually realize how similar he is to the antagonist whom we abhor.

Note on the ending: The hypnotist is almost certaibly paid off by the antagonist to give Oh Dae-su a failed hypnotism. She says the monster walked away, yet it is revealed that Oh Dae-su was the one that walked, meaning he retains the memory and left his old self behind. He also smiles creepily at the end and devolve into a painful cry. The antagonist wanted to make sure Oh Dae-su remembers.

Anyway, disclaimer, art is interpretive and this analysis is just as influeced by my own morals as is any other.