O'Neal was unable to travel with his teammates to Canada on Saturday because his passport was either lost or stolen, Warriors coach Mark Jackson said.
"Misunderstanding," Jackson said. "We thought we had it. The ball was dropped. We don't know whether it was stolen or lost or what have you, but we discovered it late afternoon yesterday and there was not enough time to fix it and get him here."
After beating the Knicks in New York on Friday night, Golden State flew north of the border on Saturday.
Athletes occasionally run into difficulty clearing Canadian customs. Baltimore Orioles outfielder Adam Jones was one of two players detained for extra questioning before a May 2010 series against the Blue Jays.
Jones, who has the same name as oft-troubled former NFL player Adam "Pacman" Jones, said it was case of mistaken identity after immigration officials told him he'd been red-flagged because he allegedly had a criminal record.
"I don't know if it's him," Jones said at the time. "We've got the same name, obviously, but I don't know if it's him."
In April 2010, the major league baseball players' union warned members with criminal convictions or arrests to contact the association before trying to enter Canada.
Toronto is the fourth stop on Golden State's current six-game road trip. O'Neal will rejoin the Warriors when they visit Indiana on Tuesday.
O'Neal played 41 games for Toronto in the 2008-09 season before being traded to Miami, so he's no stranger to taking his passport on road trips.