Two truck drivers who were making deliveries were injured, GM spokesman Dan Flores said.
One of the truck drivers was sent to a Midwest City hospital, but the extent of his injuries was unknown, Flores said. The other suffered minor injuries and was treated at the plant.
None of the 1,000-plus employees inside the plant was injured, Flores said.
Gov. Brad Henry said damage to the GM plant will have a significant impact on the state and our economy.
"They are just very, very excited by the fact that nobody was seriously injured," Henry said. "It's miraculous," Robert D. Jones, GM plant personnel director, said of the low number of injuries.
A warning siren sounded at the plant about 20 minutes before the twister hit. Workers took shelter in a concrete-lined room in the plants old paint shop.
Flores said plant officials were having difficulty evaluating the damage without electricity.
GM spokeswoman Kathy Oden said the plant will not produce vehicles today, but some employees will be securing the dam aged plant.
At a United Auto Workers union hall just west of the GM plant, about two dozen people were attending a pre-retirement seminar when the tornado struck.
"It sounded like gravel hitting a window and plywood being torn up," Lori Johnson, 42, of Oklahoma City, said.
The leader of the seminar was monitoring a TV broadcast as the tornado approached.
"He told us to get down. He said 'here it comes,' and we heard it coming," said seminar participant Leola Goodridge, 63, of Ok lahoma City.