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 ... I, I, I. In other words, it sounds like he's talking about what HE did.

And then the bombshell ... he says "Charlie" told him he wouldn't believe what was going on at Nicole's house. Simpson doesn't say what he was told was going down, but added, "It's gotta stop" ... and that was the impetus for the drive over to Nicole's.

As we reported, Simpson says in the show "Charlie" was the accomplice and went with the killer in the Bronco over to Nicole's Brentwood condo. He says the killer saw candles in Nicole's window, looking like a man was coming over and suddenly Ron Goldman appeared. Simpson says the killer started screaming, Nicole came to the front door, the killer blacked out and when he came to he was covered in blood.

The special airs Sunday at 8 PM on FOX.