Samson Ray Buck, 42, a Democrat and also a rancher, will run for the House District 49 seat that was held by Hyman, D-Leon, said Ivan Holmes, Oklahoma Democratic Party chairman.
Buck lives on a ranch on the Love-Carter county line.
"First and foremost, my condolences go out to the entire Hyman family,” Buck said in a statement.
"Terry Hyman was a true statesman and friend to southern Oklahoma. As my neighbor, Mike Steele, who was a close friend of Rep. Hyman, said yesterday, District 49 will not be able to replace Terry Hyman.”
Hyman, 56, was killed on his Love County farm in an accident involving his tractor, the Oklahoma Highway Patrol said.
He was unopposed for the Democratic nomination and had a Republican challenger for the general election.
Hyman's funeral will be at 2 p.m. today.