The boy and his sister have been staying with family friends while their parents began cleaning up in Moore after their home was destroyed by the EF-5 twister May 20.
Garland County deputies say the 5-year-old was attacked at a home near Jessieville around noon Sunday by a Bullmastiff estimated to weigh between 150 and 170 pounds.
The owner told authorities she tried to pull the dog off the boy but by the time she got the two separated the boy had already sustained major lacerations.
"The child was putting on some shoes and was crying and upset and the dog possibly took that as being aggressive...," said Scott Hinojosa with the Garland County Sheriff's Office. "... at that time the child was attacked."
The boy was taken to a local hospital where he succumbed to his injuries.
Deputies say the dog ran away after the attack and hasn't been located.