Gary Paul Karr, 52, is charged in federal court with conspiracy to kidnap and extort money from O'Hair, her son Jon Garth Murray and her granddaughter Robin Murray O'Hair, whom she had adopted as her daughter. If convicted, Karr could face life in prison.
Prosecution and defense attorneys began interviewing a pool of 250 people from which they will select 15 jurors for the trial, expected to last three weeks. Opening arguments were to follow.
``We are going to have several days of defense evidence,'' said Karr's defense attorney, Tom Mills of Dallas.
Five years after O'Hair and her family members vanished along with $500,000 in gold coins, federal authorities finally believe they know what happened to her.
Though no bodies were ever found, prosecutors say the three were dismembered and buried.
``We do believe Mr. Karr is a very dangerous individual,'' U.S. Attorney Gerald Carruth said earlier.
O'Hair is best known for launching the legal challenge that led to the 1963 Supreme Court decision that effectively struck down organized prayer in public schools as unconstitutional.
In 1995, she and her two grown children vanished from San Antonio along with the coins. She wasn't reported missing until a year later when her estranged son, William Murray, called Austin police.
Initial speculation was that the three were victims of foul play or merely ran off with the money from their organization, United Secularists of America. Some suggested O'Hair, who was 77 and ailing, had gone off to die quietly to pre-empt Christians who might have prayed for her soul.
``I think the evidence is just as consistent with them having done what they said they were going to do - leave town and get away from the IRS,'' said Karr's attorney, Tom Mills of Dallas. ``We're just going to have to see what evidence there is that some violence happened to them.''
According to court records, investigators believe the family was killed, dismembered in an Austin storage unit, then put into 55-gallon drums and buried.
Despite last year's searches of a South Texas ranch where an informant said the drums were taken, investigators have found no bodies.
Investigators have alleged that Karr, O'Hair's former office manager David Roland Waters, and Florida resident Danny Fry kidnapped the family and forced them to turn over the gold coins.
Karr, who has a violent criminal history dating back to the 1960s, has said the O'Hairs voluntarily left Austin with Waters, his former cellmate in Illinois.
In September 1995, authorities say, Karr went with Jon Murray to New Jersey to arrange a $600,000 bank transfer. About a week later, Murray took delivery of approximately $500,000 worth of gold coins.
Karr and Waters were arrested last year on weapons charges.
Waters has not been charged in the alleged plot. He is serving up to 60 years in prison for the weapons charges and a previous conviction for stealing from O'Hair's atheist organization.
Fry was found dead in 1995 in Dallas. His body wasn't identified until three years later because his hands and head had been cut off.