Taloa Foster, 33, was charged with child endangerment.
Witnesses said the toddler grabbed the wheel and steered the truck across lanes of traffic after his mother fell from the vehicle on Wednesday.
The boy's twin brother was also in the truck at the time.
The vehicle eventually crashed into an embankment along the road near Ada, about 80 miles (130km) southeast of Oklahoma City.
The boys were unhurt, police said.
Police said Ms Foster told them that the boy had unbuckled his seat belt and she was trying to secure him when she fell out of the vehicle.
An Oklahoma woman has been arrested after police say she drunkenly fell out of her pickup truck and her three-year-old son steered the vehicle to safety across a four-lane road.