Bob Stoops explains why this was the right time for him to step down as Oklahoma head coach and expects a smooth transition for new coach Lincoln Riley and the players. (1:58)

Bob Stoops, the longest-tenured coach in college football, has stepped down after 18 seasons with the Oklahoma Sooners, he announced Wednesday.

"Stepping down after 18 years as the head football coach here at Oklahoma," Stoops said at a Wednesday afternoon news conference. "I feel like I've been absolutely the luckiest, most fortunate guy in the world and incredibly blessed to have experienced what we have over the last 18½ years. I felt I just didn't want to miss the right opportunity to be able to step away and hand this baton off to Lincoln Riley and to help this all just keep going in a great direction."

Riley, 33, will become the youngest head coach at the FBS level after two seasons as OU's offensive coordinator. Stoops will remain as special assistant to the athletic director.

"The time is now because Lincoln Riley will provide a seamless transition as the new head coach, capitalizing on an excellent staff that is already in place and providing familiarity and confidence for our players," Stoops said in a statement released by the program earlier Wednesday. "Now is simply the ideal time for me and our program to make this transition."

Stoops said he did not base his decision on health reasons and that he has "had no incidents that would prevent me from coaching." A high-ranking Oklahoma athletic department official told ESPN's Gene Wojciechowski that Stoops' decision "was not an overnight revelation."

The source added that Stoops' father, Ron, suffered a heart attack and died while coaching a high school game in Ohio in the late 1980s and that Stoops has always had that in his mind. The source also said Stoops didn't want to get to the point at which the school forced him out, adding that he doesn't expect Stoops to coach again at any level.

"He didn't want to go from the sidelines to the graveyard," former coach Steve Spurrier told ESPN on Wednesday. "He pretty much was able to go out on top."

Stoops joined the Sooners as a rookie head coach in 1999, and he guided OU to the 2000 national championship, 10 Big 12 titles and 14 seasons with double-digit wins, including an 11-2 mark in 2016.

Bob Stoops is the all-time winningest coach at Oklahoma, compiling a 190-48 record and taking the Sooners to bowl games in all 18 of his seasons. Ben Queen/USA TODAY Sports

Stoops, who turns 57 on Sept. 9, is the winningest coach in OU history, with a 190-48 record (.798 winning percentage). He has gone 121-29 in Big 12 play and has taken the Sooners to bowl games in every one of his 18 seasons -- going 9-9, capped by January's 35-19 victory over Auburn in the Allstate Sugar Bowl.

His 190 victories had ranked fourth among active FBS coaches, behind Brian Kelly (230), Nick Saban (205) and Bill Snyder (202).

Only four times in Stoops' career did the Sooners win fewer than 10 games, and he never had a losing record.

In addition, he coached two Heisman Trophy winners: quarterbacks Jason White (2003) and Sam Bradford (2008).

"The Bible says, 'To everything there is a season, and a time to every purpose under the heaven,'" Stoops said in his statement. "I'm grateful for this season of my life, and feel I've fulfilled my purpose here at OU as its head football coach."

Stoops was finishing his third season as defensive coordinator under Spurrier at Florida when the Sooners came calling and hired him in December 1998.

Oklahoma had fallen into disrepair after three losing seasons under coach John Blake. When Stoops arrived, chicken bones littered the practice field, where fans had still been allowed to tailgate. Even more telling, the sign longtime coach Bud Wilkinson put up coming out of Oklahoma's locker room, "PLAY LIKE A CHAMPION TODAY," was dilapidated and missing letters.

After only two seasons with "Big Game Bob" at the helm, Oklahoma was a national champion again, beating No. 2 Florida State 13-2 in the BCS National Championship Game at the Orange Bowl following the 2000 season.

The Sooners would play for the BCS national title three more times under Stoops -- losing to LSU (2003 season), USC (2004) and Florida (2008) -- and earning a spot in the College Football Playoff in 2015.

Boomer Sooner The Oklahoma Sooners have had the second-best winning percentage in the FBS since Bob Stoops took over as coach in 1999: Teams Win pct. Boise State .838 Oklahoma .798 Ohio State .795 LSU .740 TCU .739

His 2008 team scored 716 points, which at the time had been the most by any team in a season.

"Bob did a tremendous job turning things around at Oklahoma and putting their program back in national prominence," former Texas coach Mack Brown said in a statement. "Our rivalry game became a focal point of college football every year and was great for both schools, the Big 12 and college football as a whole.

"He will be remembered as a Sooner legend. I wish him the best."

Stoops went 11-7 against the rival Longhorns during his tenure.

This upcoming season's entry into the Red River Rivalry will feature the first matchup between first-year head coaches at both programs since 1947, as Tom Herman is taking over at Texas after a successful run at Houston.

Stoops was named Walter Camp Coach of the Year in 2000 and 2003 and was named Big 12 Coach of the Year six times. He is the only head coach in the BCS era to have won the Rose Bowl, Orange Bowl, Fiesta Bowl and Sugar Bowl.

On Wednesday, Stoops took time to thank his players -- past and present -- and the fans.

"I'm especially thankful for being able to coach so many talented young men over my 18 years here," Stoops said in his statement. "It has been so rewarding to see these players come to OU and mature over a four- or five-year career, and not just on the field. To play a small part in their growth is what I will always cherish the most.

"None of my success would have happened without the best fans in the country. I can't tell you how much I appreciated the 110 consecutive home sellouts. The passion of our fan base is unmatched, and their support has played a huge role in not only home games, but road games and all 18 of our bowl games, as well."

@OU_CoachStoops Thank you for everything Coach. You turned a little boy's dreams into a reality. I will be forever grateful. #ThanksBob — Baker Mayfield (@baker_mayfield6) June 7, 2017

If you are wondering how @OU_CoachStoops' current and former players feel about him...just look at my face in this picture. Will be missed. pic.twitter.com/cJTUFxQM2w — Gabe Ikard (@GabeIkard) June 7, 2017

For all the wins, all the titles, all the memories, and all the lives you touched along the way, #ThanksBob https://t.co/StF2uClKjf — Oklahoma Football (@OU_Football) June 7, 2017

Iowa coach Kirk Ferentz, who was hired by the Hawkeyes just one day after the Sooners hired Stoops, becomes the new longest-tenured coach in the FBS.

Over the years, Stoops' name has been attached to many college and NFL coaching vacancies. Each time, however, Stoops chose to stay with the Sooners.

"I'm thankful that my career at Oklahoma was marked with consistent leadership in president David Boren and director of athletics Joe Castiglione," Stoops said in his statement. "It's extremely rare in college athletics to have no change in these leadership positions over a nearly 20-year span. I always appreciated the way both of them supported me and our program. They both played an enormous role in all our successes."

Both Boren and Castiglione spoke at Wednesday's news conference, with Boren thanking Stoops for "a job exceptionally well done."

Big-game Bob Since 1999, only one coach has had more wins against AP top-10 opponents than Oklahoma's Bob Stoops: Coach Wins* Nick Saban 31 Bob Stoops 21 Urban Meyer 19 Les Miles 18 Tommy Tuberville 17 * vs. AP top-10 foes since '99

Stoops attended Iowa and was a four-year starter at defensive back. His first coaching position was as a volunteer coach and graduate assistant for the Hawkeyes. That was followed by assistant coaching positions at Kent State, Kansas State and Florida, where he was part of the Gators' national championship team in the 1996 season.

He is part of a coaching family. His brother Mike is the Sooners' defensive coordinator and was head coach at Arizona from 2003 to 2011. His brother Mark has been head coach at Kentucky since 2012. His brother Ron is special-teams coach at Youngstown State.

"Bob truly represents what is good about college football, and the success of his career speaks for itself," Mark Stoops said. "What he means to me as a brother and a coach is immeasurable."

Stoops also groomed several assistant coaches who moved on to bigger things, including Mike Leach (Washington State), Kevin Sumlin (Texas A&M), Bo Pelini (Youngstown State) and Jay Norvell (Nevada).

"Bob is a tremendous mentor to so many players and coaches, including myself," Sumlin said in a statement. "Not only is he a great mentor to me, but he and his wife, Carol, have been great friends to Char and I for many years. His success and longevity speak for themselves. He will be a success in whatever he pursues."

Stoops, who has two sons who play high school football and are entering their senior year, said he and his wife plan to stay in Norman, Oklahoma.

"The coaching life is like a relay race, and I'm thankful for my turn and am confident as I pass the baton," Stoops said in his statement. "Carol and I intend on staying in Norman -- it is our home. I will be available to Coach Riley and the athletic department in any manner."

Spurrier offered additional comments on Stoops.

"Bobby is ready to go to the beach and watch his kids play ball," Spurrier told ESPN. "He's had a wonderful career. He knew when enough was enough."

Riley takes over a team that will again be the favorite to win the Big 12 with a Heisman Trophy finalist in quarterback Baker Mayfield.

"The hardest moment coming to terms with this was seeing my players just a little while ago," Stoops said at Wednesday's news conference. "As they all know and will tell you, I love them dearly. Look at them all as your own children.

"I also know this is going to be great for them moving forward, so that made it easier."

Riley's first game will be at home against UTEP on Sept. 2. His second game will be at Ohio State.

Last month, Riley, who won the Broyles Award as the nation's top assistant coach in 2015, was given a three-year contract extension worth $1.3 million per year, making him one of the highest-paid coordinators in the country.

"I'm sincerely honored to be given this opportunity to be the head football coach at the University of Oklahoma," Riley said in a statement. "I want to thank Coach Stoops for bringing me here two years ago and making me part of the Sooner family. He is one of the greatest coaches in the history of the game, at any level. I'm absolutely thankful for our friendship and for the mentorship he has provided.

"Coaching at Oklahoma is a dream come true for me and my family. ... I look forward to continuing the tradition of excellence that Coach Stoops and so many others before him have instilled in this great program."

The Sooners have fared well with Riley as offensive coordinator. During his two seasons at OU, the Sooners were one of just two FBS teams -- along with TCU -- to average more than 300 yards passing (313.1) and 200 yards rushing (229.4) per game.

Riley grew emotional when addressing Stoops at Wednesday's news conference.

"He gave me a chance a few years ago I'll never forget," Riley said. "His guidance has been incredible. To be the guy to take over for him is an incredible honor."

The Associated Press contributed to this report.