The shooting occurred around 6:30 p.m. Saturday near the entrance to a department store, said Oklahoma County sheriff's spokesman Mark Myers. The motive for the shooting was not known and there were no other injuries, he said.
Ladra Morton, 13, told The Oklahoman he was shopping with friends when he heard gunshots.
"You could hear them yelling gang stuff during the fight," Morton said.
Sheriff John Whetsel said authorities found no indication of gang involvement during their preliminary investigation.
The deputy, who was working as a security guard at the Crossroads Mall, was taking a break when "he heard a commotion downstairs, kind of like a couple of guys fighting, then he heard a gunshot," Myers said.
The deputy, whose name was not released, went to a railing and looked down to the first floor of the mall where he saw a young man with a gun.
"He turned around with the gun, our deputy shot him, killed the person with the gun," Myers said.
The wounded boy was taken to a nearby hospital in fair condition, according to ambulance company spokeswoman Lara O'Leary.
The identities of the two youths were not immediately released.
Oklahoma City police have been asked to assist in the investigation into the shooting, Myers said.
The mall closed about an hour early, said spokeswoman Christi Parks.