After Oklahoma's win over Ohio State on Sept. 9, Bob Stoops was beaming as he waited to greet the triumphant Sooners at the entrance to the visiting team's tunnel.

The longtime Oklahoma coach, who retired in June after 18 seasons, put his arm around his replacement, Lincoln Riley, and whispered some words in confidence after his protégé's first signature win.

"It was emotional," Riley said after the game. It was only his second game as a head coach, but the win validated his selection as Stoops' successor and set the Sooners on their path to the College Football Playoff.

Lincoln Riley, right, has Oklahoma in the CFP in his first season taking over for Stoops. AP Photo/Sue Ogrocki

Stoops was also there for Riley's first devastating loss. On Oct. 8, a day after the Sooners suffered a shocking 38-31 upset to unranked Iowa State on their home field, Stoops called Riley and told him one of his favorite anecdotes. A trap had been set to stop a monkey from stealing fruit. The monkey reached into the trap to get a piece a fruit and got stuck. All it had to do was drop the fruit and it would be released, but the monkey wouldn't let it go.

"Don't hold on to it and let it beat you again," Stoops told Riley. "If you keep hanging on to something, you've got an opportunity to keep getting hurt by it."

"He probably helped me get past it faster than I would have been able to do on my own," Riley said.

The rookie and the veteran helped each other through the entire season, and the result was a coaching transition rarely seen. Stoops' decision to retire on June 7 at age 56 and make Riley, then 33, the youngest head coach in FBS blindsided just about everyone who follows college football.

However, the decision was far more calculated and thoughtful than it appeared at the time. Stoops considered the leadership on the team and knew that this group of players, led by Heisman Trophy-winning quarterback Baker Mayfield, "would handle it well." He considered Riley to be "more than ready for this." He considered the timing, knowing that giving Riley the entire summer would allow him to make any adjustments before the season began while continuing to prepare for the early signing period in December.

"Way back when it happened, everybody was all, 'What's the matter here?'" Stoops said. "Now they're realizing that actually there was nothing wrong, it was just the perfect time for it all."

OU athletic director Joe Castiglione called it "one of the most incredible selfless acts of a leader" he's seen.

"It would've been very easy for him to stay on another year or any other period of time," Castiglione said. "It was really important for him to have a good team to turn over to the next head coach so that the next head coach would have a great beginning to put their stamp on it, to really continue the momentum that already exists. He thought about every single element of this, including his own reasons for stepping away from coaching."

Bob Stoops was there to congratulate Lincoln Riley after Oklahoma beat Ohio State on Sept. 9. Joe Maiorana-USA TODAY Sports

It worked because of the trust and respect involved at every level in the department, including Castiglione and OU president Dave Boren, and because Riley won more than he lost. Had Riley not beaten Ohio State in Week 2, then refocused the team after the Iowa State loss, this improbable handoff wouldn't have resonated the way it does now, just two wins from a national title.

"I don't think this could happen many places," Riley said. "The people who are supposed to be making the decisions within a university system and athletic department, this is just an incredibly stable place for a lot of reasons, and I think all of that together, and plus everybody's willingness to put aside what was best for them personally and just work together on it, has been a big factor in it going the way that it went."

Stoops said he was asked by some to consider a coach-in-waiting plan for a year but was opposed to it.

"I didn't feel that would be fair and really empower Lincoln like this has," Stoops said. "I felt, with this team, this would really empower him to show the country and all of these recruits that nothing's going to change."

Very little did.

OU kept winning, and Stoops kept watching. On Mondays during the season, Stoops sat quietly through about half of the quarterbacks' meetings to see what corrections they were working on, and he would sometimes stop by on Wednesdays to see which wrinkles the offense was adding that week.

"I don't say anything," Stoops said. "I'm not coaching whatsoever, and nobody is asking me what I think. I'm not sitting there critiquing him whatsoever. It's been fun. There isn't any looking over the shoulder. [Riley] doesn't need any opinions. He's probably also just gracious enough to know I need to be around a little bit. I felt very much the same way when I arrived here; I always loved it when Coach [Barry] Switzer was around. It was never threatening."

Riley agreed.

"I think both of us are just secure enough in who we are and what we have here," he said. "I know for me, I understand that I wouldn't be here without him. This program wouldn't be where it is without him. So I've always looked at the chance to have him around as a positive. It's never felt awkward or any of that to me. It helps me. I think it's great for all of us to have him around. We want him around. And he's handled it well, too. He's not anybody who would ever try to overstep it or anything like that."

Nor did he have to.

Stoops hired Riley as his offensive coordinator in 2015, and he quickly became one of the most coveted coaches on the market. Whenever an opportunity presented itself, Riley said he was always up front with Stoops and Castiglione and valued their opinions on how he should weigh his options.

"There were a few that were difficult," Riley said of his offers. "I was very content here in a lot of ways. I certainly wasn't looking to leave, so some of the opportunities that came about, I eliminated them pretty quickly because I loved where I was at. I felt like we had a chance to win and to win a national championship here. I loved working for Bob and his staff. My family loved it here. I certainly wasn't in a hurry."

Stoops and Castiglione had talked to Riley about the value of staying at OU but never made any promises other than increasing his salary and extending his contract as their coordinator.

"Coach Stoops and I and President Boren kicked around the possibility of Lincoln Riley being our head coach someday down the road," Castiglione said.

They just never told him about it.

"Nothing was absolutely ever promised, not even close to it," Riley said, "But I think [Castiglione] just really stressed more than anything that we had a great situation here, and be patient, and if I did leave, it would be something that was a no-brainer type of opportunity, and they felt like we had something going good here and would for a long time."

For 18 years, Oklahoma had something good going. 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.

Seldom, if ever, does a coach as tenured and accomplished as Stoops know when to quit.

"When is there a good time?" Stoops said. "You come to that realization, too, that if not now, then when? You start asking that question honestly, I just in my mind strongly felt that this was the best time."

He still needs the program, though, as much as it needs him. Stoops' official title is special assistant to the athletic director. (Stoops laughs when he says he and Castiglione are both still trying to figure out what that means.) Castiglione said they knew that when Stoops retired he would stay in Norman, so they wanted to give him time to "adjust and to some degree decompress."

"It's the first time he hasn't led a team in almost 19 years," Castiglione said. "He's been in the game to a large degree for the better part of his life. We knew that would be something he would adjust to on his own pace and in his own way. But we could also benefit a great deal by having him here. We consider him as much a part of our success as anybody. Not just the past success but what's taking place right now."

Riley laughed a little when he conceded he doesn't even know where Stoops' office is. (It's actually just a makeshift space in one of the suites at Memorial Stadium that overlooks the field.) Stoops went to every home game but one -- Iowa State. That weekend, he took his son, Drake, to Ohio for an official visit and didn't get home until Sunday. It was the first home game Stoops had missed in 19 years.

Whenever Riley needed his predecessor this fall, though, Stoops was there.