LAS VEGAS — A second co-defendant in the O.J. Simpson armed robbery case said yesterday that he will plead guilty to a reduced charge and testify against Mr. Simpson and four others in the alleged hotel room theft of sports collectibles from two memorabilia dealers.

Walter Alexander, 46, of Mesa, Ariz., 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. Charles Cashmore, 40, of Las Vegas, told the same judge that he would plead guilty to a felony and testify for the prosecution.

“It’s always a prosecutor’s strategy to go after the little fish to get to the big fish,” Cashmore’s attorney, Edward Miley, said outside court. “In this, it seems to be that O.J. Simpson is the big fish.”

The plea agreements up the ante in the prosecution of Mr. Simpson. 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. “He can establish who was in the room, what was said, who had guns, who didn’t have guns, potentially who may have seen guns, who didn’t see guns,” Mr. Miley said. “I think he wishes he would have never met O.J.”