"I would just ask all Hoosiers to join hands and say a prayer," said Lt. Gov. Joe Kernan, who assumed some of O'Bannon's duties but was not officially named acting governor Monday.
O'Bannon, 73, a Democrat in his second term, was found Monday morning in his Chicago hotel room. He was unconscious, unresponsive and near death when he was rushed to the hospital, doctors said.
"This is up to Mother Nature and his own ability to bounce back, his own brain's ability to recover," said Dr. Hunt Batjer, chairman of neurological surgery at Northwestern Memorial Hospital.
Doctors said it was too early to predict the likelihood of O'Bannon fully recovering. The outcome depends on how much bleeding occurs, how much of the brain is affected, and how long the patient goes without treatment.
The Indiana Constitution allows for a transfer of power from the governor to lieutenant governor if the governor is incapacitated. However, that requires a petition to be filed by the president pro tem of the Senate and the speaker of the House and a ruling by the Indiana Supreme Court.
House Speaker Patrick Bauer said he and Sen. President Pro Tem Robert Garton were prepared to draft the necessary paperwork. The Supreme Court justices were being assembled.
"I think we're all just hoping and praying at this point for a quick recovery for the governor, hoping this situation is not as serious as is currently indicated," state House GOP Leader Brian Bosma said.
O'Bannon had been staying at the Palmer House Hilton in Chicago to attend a conference of the Midwest U.S.-Japan Association. The governor was found in his pajamas lying on his back in a closet in the hotel room.
Neurosurgeon Wesley Yapor said surgeons removed blood from both sides of O'Bannon's brain. The doctor said some of the blood probably came from an injury, suggesting that the governor fell after becoming ill.
After the stroke, O'Bannon's wife, Judy, arrived at the hospital from Indiana along with their daughter, Jennifer, and their other adult children. Kernan, who was also at the Chicago conference, returned to Indianapolis.
Strokes are the nation's leading cause of disability in the United States and the No. 3 killer. Most are ischemic strokes, caused when arteries feeding the brain are blocked, but some are caused by bleeding in the brain.
Stroke survivors can suffer brain damage and often suffer permanent disability including paralysis, loss of speech and vision, and weakness.
In his seven years as governor, O'Bannon, a moderate known for his grandfatherly charm, has forged alliances to reform education and try to improve the state's economy.
The governor coasted to re-election in 2000, but his popularity began to slide in his second term as the economy faltered. He is barred by term limits from running again next year.
"It will be a real shock if he doesn't recover," said Kim Self, sales manager at Magdalena's Restaurant & Gourmet Gift Shop in the southern town of Corydon, O'Bannon's home.
The last time Indiana had an acting governor for any length of time was in 1924. Emmitt Branch took office April 30, 1924, when Gov. Warren McCray was forced to resign after being convicted of mail fraud.