Walter Alexander told a judge he will plead guilty to conspiracy to commit robbery, a felony. He could face between one and six years in prison.
Outside the courtroom, Alexander and his lawyers declined to say what testimony he will provide.
Earlier Monday, Charles Cashmore told the same judge that he would plead guilty to a felony and testify for the prosecution.
The plea agreements add some tension in the prosecution of Simpson. Cashmore can testify that guns were involved in the Sept. 13 confrontation with two sports memorabilia dealers at a Las Vegas casino hotel room, his lawyer said.
Simpson and his lawyers have denied guns were in the room at the Palace Station hotel-casino. His lawyers did not respond Monday to requests for comment.
Both Alexander and Cashmore waived their preliminary hearings. Las Vegas Justice of the Peace Joe Bonaventure Jr., set both men's arraignments for Oct. 23.
Cashmore faces up to five years in prison.
Outside court, Cashmore said he thought he'd done the right thing, but declined additional comment.
Cashmore was initially arraigned on nine felonies and a gross misdemeanor, charges that included kidnapping, armed robbery, assault with a deadly weapon and conspiracy. A kidnapping conviction alone could have resulted in a sentence of life in prison with parole.
Simpson and the others, Clarence "C.J." Stewart, Michael McClinton, and Charles Ehrlich, are due in court for a preliminary hearing on Nov. 8 and 9. Bonaventure will decide then whether there is enough evidence to send the case to trial in state court.
If asked, Cashmore will testify if asked at the preliminary hearing that Alexander and McClinton were armed when they entered the room with Simpson, Miley said.
Simpson claims at least some of the items taken from collectors Alfred Beardsley and Bruce Fromong belonged to him, and his lawyers have maintained that no guns were used. Simpson and the others are charged with kidnapping, armed robbery, assault, burglary and conspiracy.
Cashmore was introduced to Simpson and most of the others in the group for the first time minutes before the alleged robbery, Miley said.
In 1996, he plea bargained a felony theft charge to a misdemeanor and received probation in an embezzlement case in Utah.
Alexander's lawyer, Robert Dennis Rentzer, declined to say whether Alexander had a gun in the room, but expressed doubt Cashmore could say Alexander was armed.
McClinton's lawyer, Bill Terry, did not respond to requests for comment.
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