A couple of answers that may be useful to understand this film better!!

Rating: 9/10 (12 February 2008)

Hi! I am a South Korean national student here in London, and would like to give you some clues why the main character, Oh Dae-Soo, was kept in the private prison for exactly 15 years and why he ate the live octopus as I hope you all understand this film a little better.
Firstly, his wife was killed one year after his being kept in the privately run prison. Obviously, Oh Dae-Soo didn't kill her but was suspected of murdering her. It must be done by Lee Yu-Jin who planned to have his sister's revenge upon Oh Dae-Soo. In South Korea, criminals of unsolved crimes including murder cases are forgiven/overlooked by law 15 years after the crimes were committed, which means a murder suspect wouldn't be charged or get punished unless the suspect is caught within 15 years after the murder. This is why Oh Dae-Soo was released 15 years after his wife was killed. Thus, he wouldn't be disturbed by the police investigation and Lee Yu-Jin could play with him at his own discretion.
Secondly, nobody in the country eats live octopus like the way he ate in the film. It seems to me that the scene shows his uncontrollable anger against the man who he talked with on the phone and didn't say anything about the reasons why he was kept in the prison. If there was a piece of cake or a bowl of rice in front of him, I guess he would probably eat it in the same way he did with the octopus. So, please don't get it wrong that eating something alive is part of Korean culture. It should be understood itself in that extreme context.
I saw this film several times with my friends. I didn't quite understand it in the first view but came to like it in the second time and it's one of my favourites now. I like the complicated plot and the shocking ending. As I'm studying translation of English and Korean, most of my classmates and friends are English who spent some time in South Korean and they liked this one the most among other Korean films.
I hope my talk helps.