The most likely reason is the long weekend just mentioned…competition over the long Turkey Day holiday period in the States is not very fierce in the cinema this year, with the only major competition in adult demographics coming in the form of the James Franco/Jason Statham starring Homefront. If Lee’s Oldboy would have remained in the October slot, it would have had to duke it out with both Ridley Scott’s The Counselor and a new Jackass film.
A new Jackass film? I didn’t know they were doing another one…I simply thought that the Jackass crew had long been relegated to the DTV market. Still, if you’re trying to compete in the R-rated market, the less competition, the better. And this move from the production studio also likely takes into consideration the limited viewing memory of the Academy members; the closer to the end of the year, the more consideration Oldboy make get come nomination time.
Source: Firstshowing.net