Democrat Paula Sophia will face businessman and former pastor Jason Dunnington in the Aug. 26 runoff.
The winner will take a seat at the Legislature as there is no Republican in the race.
Sophia used to patrol the district she seeks to represent and retired from the police force in April to run for the Legislature.
She is a U.S. Army veteran of Desert Storm who joined the Oklahoma City Police Department in 1992.
She was engaged in a brief legal battle with the city after transitioning in 2001, but ultimately returned to the force.