O.J. Simpson's 'Confession' on Fox Special 'If I Did It'

To Murdering Nicole

This never-before-seen video is the smoking gun in the upcoming FOX special, 'If I Did It' ... and it provides the reason O.J. Simpson went to Nicole Brown Simpson's house the night she was murdered.

Simpson is interviewed by book publisher Judith Regan, who collaborated with O.J.on the book, 'If I Did It.' The premise is O.J. Simpson's "hypothetical" on how Nicole Brown Simpson and Ron Goldman were slaughtered on June 12, 1994.

O.J. begins by saying how "difficult" it is for him to talk in the hypothetical ... weird and somewhat incriminating.

Then he talks about the murderer's friend -- "Charlie" -- and suddenly Simpson goes from talking in the hypothetical to first person…