LAS VEGAS — A co-defendant in the O.J. Simpson armed robbery case told a judge today he would plead guilty to a felony and testify against Mr. Simpson and four others in the hotel room theft of sports collectibles from two memorabilia dealers.

The plea agreement with Charles Cashmore, 40, of Las Vegas, ups the ante in the prosecution of Mr. Simpson. Mr. Cashmore can testify that guns were involved in the September 13 confrontation with two sports memorabilia dealers at a Las Vegas casino hotel room, his lawyer said.

Mr. Simpson and his lawyers have denied guns were in the room.

“In District Court, he’ll be pleading guilty to accessory to robbery,” a Clark County district attorney, David Roger, told the judge. “He’s agreed to provide truthful testimony.”

Mr. Cashmore waived his preliminary hearing. A Las Vegas justice of the peace, Joe Bonaventure Jr., set arraignment for October 23. He and the prosecution did not agree on a possible sentence before today’s hearing, said Mr. Cashmore’s lawyer, Edward Miley. He faces up to five years in prison.

Mr. Simpson and the others, Walter Alexander, Clarence Stewart, Michael McClinton, and Charles Ehrlich, are due in court for a preliminary hearing on November 8 and 9. Mr. Bonaventure will decide then whether there is enough evidence to send the case to trial in state court.