Another reason they might not want to mention just yet is Spike Lee recently coming under fire for moving to Kickstarter to try and fund his next project. The studio probably figures an extra month to let people forget about it might not hurt.
Personally I don't see a reason for the move. The only competition Oldboy had was Paranormal Activity 5 and that got pushed back to January. Oh well. I don't really want to see it anyway.