NORMAN, Okla. (AP) — Oklahoma coach Bob Stoops abruptly announced his retirement Wednesday, a stunning offseason move by the 56-year-old future Hall of Famer who led the Sooners to 11 conference championships and a national title in 18 seasons.

Stoops was the longest-tenured active coach in major college football, taking the job at Oklahoma a day before Kirk Ferentz started at Iowa. Stoops was 190-48 with the Sooners, his only college head-coaching job.

“I understand there has been some speculation about my health,” Stoops said in a statement issued two hours before a campus news conference. “My health was not the deciding factor in this decision and I’ve had no incidents that would prevent me from coaching. I feel the timing is perfect to hand over the reins.”

Offensive coordinator Lincoln Riley, 33, is being promoted to head coach, making him the youngest head coach in FBS. Just last month, Oklahoma gave Riley a three-year contract extension worth $1.3 million per year, making him one of the highest-paid coordinators in the country.

Stoops was 39 when he left Florida, where he was defensive coordinator under Steve Spurrier, to take over at Oklahoma in 1999. The storied program had struggled for more than a decade to replace Barry Switzer. Stoops quickly returned Oklahoma to national prominence, winning a national championship in his second season with a win over Florida State.

His early success that included three straight victories in major bowls and a five-game winning streak against Texas earned him the nickname “Big Game Bob.”

While Stoops was not able to bring another national championship to Norman, he did lead Oklahoma to three more BCS championship games and a spot in the College Football Playoff two years ago.

The shortcomings in championship games and lack of another national title frustrated some fans. Oklahoma lost consecutive BCS championship games in 2003 and ’04 and dropped another title game in 2008.

Big Game Bob wasn’t delivering the biggest wins, but the program was as consistent a national power as any in the country during his tenure. Only four times in Stoops’ career did the Sooners win less than 10 games, and he never had a losing record.

The son of a football coach from Youngstown, Ohio, Stoops is one of four brothers who became college football coaches. His father, Ron, died at 54 of a heart attack suffered while coaching a high school game.