The death toll could go higher, said Oklahoma Emergency Management spokeswoman Michelann Ooten. The tornado caused major damage in a 20-block area, she said.
"I know they are going through the rubble, trying to find people missing," she said. "There are numerous injuries."
First responders were working to free people trapped in the rubble, the department said.
At least five people died in southwestern Missouri after the storms plowed through, the National Weather Service said. Three people died after a tornado hit near Seneca in Newton County, said meteorologist Bill Davis.
Other tornadoes were reported near McAlester and Haywood in Pittsburg County and in rural Pushmataha County, both in southeastern Oklahoma.
Television footage showed some destroyed outbuildings and damaged homes west of McAlester and near Haywood. At a glass plant southwest of McAlester, the storm apparently picked up a trailer and slammed it down on garbage bins.
"These are rural areas that we are in," Pittsburg County Undersheriff Richard Sexton told KFOR-TV in Oklahoma City. "These are good people coming together at this time."
In Arkansas, a tornado damaged buildings and pulled down trees in Stuttgart. The Weather Service said that trees were down across a wide area of the southeast Arkansas city and that an elderly woman was reported trapped in her home.
In White County, the Weather Service said there was wind damage in Searcy, where trees were down in places.
The storms remained active into the evening, with watches and warnings abundant across a wide swath of the Plains and South.
Tornadoes killed 13 people on Feb. 5 and another seven were killed in an outbreak on May 2. In between was freezing weather, persistent rain and river flooding that damaged residences has slowed farmers in their planting.
