Prosecutors say Nichols helped prepare the bomb
Oklahoma City bombing conspirator Terry Nichols, who is already serving life in prison, must stand trial in state court on 160 counts of murder, a judge has ruled.
If he is found guilty, Nichols could receive the death penalty.
The judge's decision essentially means Nichols will be tried again for the 19 April 1995 bombing of the Alfred P Murrah federal building that killed 168 people and injured hundreds others.
Nichols was convicted in 1997 on federal charges of conspiracy and the involuntary manslaughter of eight federal officers in the bombing.
Local prosecutors had been pushing for Nichols to go on trial over the deaths of the other 160 people.
They also want to eliminate the possibility that Nichols could ever successfully appeal against his federal case and gain freedom.
Under the US constitution a defendant cannot be prosecuted for the same crime twice - but a state judge has already ruled that double-jeopardy protections do not apply in this case.
Nichols was a former friend of Timothy McVeigh, who was convicted and executed in June 2001 for delivering and detonating the bomb.
Nichols was at home the day the bomb exploded, but prosecutors accuse him of helping McVeigh pack the 4,000-pound (1,800-kilogram) bomb in a truck the day before the attack. They also say he helped deliver a getaway car to Oklahoma City.
Officials say the bombing of the office building was a plot to avenge the FBI siege at Waco, Texas, two years earlier.
A preliminary hearing on the new charges against Nichols is scheduled for next Tuesday.