The news broke and comparisons were made and there were several people who stated that the film caused Cho to go through with the killings. CBS in particular ran a short report on it.
However there has been no real evidence that Cho even watched Oldboy, and the professor that initially made the connection that Cho may have been influenced by Oldboy has since discounted the theory that he was influenced by the film.
Even when I first heard about this in 2007 I realized that the media was just trying to find a scapegoat to place the blame of these shootings on. I was 15 when I first saw Oldboy myself. I understood the character and why he was doing the things he was and even at that age I realized that while in some ways I agree with him, it doesn't mean I'm going to go around plotting revenge on every person who I think has done me some sort of wrong.
A violent film didn't cause Sueng-Hui Cho to go on a shooting spree, his unstable mind did. I never understood how people try to blame violent films, television, and video games as the cause for these violent outburst. Sure it's true that a suggestible mind could be triggered by seeing violence and perceiving that it's alright to do it yourself, but that person has to already be mentally unstable to arrive at the conclusion that the violent scene they see in a film is something they would want to do people in their own lives. Take it from me, I've seen so many violent scenes from so many different mediums but I never get the urge or idea to go out and reenact what I saw on screen to people who I don't like or get bothered by. And we all know that there weren't violent films and video games around in 30's and 40's when there was still violent crimes being committed (The Black Dahila murder being an example). So before we place blame on a piece of entertainment we should really analyze the individual and why they would ever be suggestible by that kind of thing in the first place.