Take a guess at which Big 12 team allowed the fewest yards per game this season.

Baylor? After all, the Bears' fast, physical defense was the biggest reason the team won 11 games.

Wrong.

TCU? Gary Patterson's defensive wizardry regularly puts the Horned Frogs at or near the top of the league.

Incorrect.

The answer may surprise you: Oklahoma.

Yes, that Oklahoma. The one that a year ago fielded a defense that, at times, served as a punchline and was porous enough that coach Lincoln Riley made a midseason coordinator change.

While the Sooners once again ride a powerful offense and Heisman Trophy finalist into the College Football Playoff semifinal at the Chick-Fil-A Peach Bowl, their defense will be the most valuable unit when they face LSU's explosive offense.

"I couldn't be happier with our defense," Riley said. "The way those guys have played with so much new; we knew it could be done here, and our guys believed it, our coaches believed it, and we got a pretty good defense here."

Under defensive coordinator Alex Grinch, the Sooners' defense transformed from liability to asset. With a focus on speed, simplicity, mentality and takeaways, Grinch led the unit from worst to first in average yardage allowed.

Choose your metric, chances are the Sooners have made dramatic improvements. Oklahoma made significant strides in points allowed per game (101st in the FBS in 2018; 48th this season), yards per play (102nd to 41st), third-down defense (119th to ninth), defensive efficiency (92nd to 34th) and defensive S&P+ (84th to 36th).

They were third in the Big 12 this season in scoring defense (behind Baylor and Kansas State), second in third-down efficiency, fourth in yards per play and fourth in defensive efficiency after finishing last or second-to-last in the league in each category in 2018.

Here's how they did it:

'Speed D'

Around the Switzer Center, the Sooners' team headquarters, defensive players can be seen roaming around the building in T-shirts with the phrase "Speed D" emblazoned across the chest.

It's a nickname that didn't originate in Norman, but dates back to Grinch's time as Washington State's defensive coordinator. It's there -- where Grinch accomplished the difficult task of fielding a quality defense opposite Mike Leach's pass-happy, Air Raid offense -- that he cultivated the identity that continues to be a staple of his defense today.

While having fleet-footed defenders helps (Grinch this spring had his defensive linemen lean up to get faster), it's not solely focused on foot speed.

"Speed D is really just about playing fast," safety Brendan "Bookie" Radley-Hiles said. "It's about knowing your responsibility from how the offense is going to line up. And once your eyes are in the correct place and you know what you're doing, you can play at a fast tempo."

To play fast, Grinch aims to simplify. The less players have to think about, the faster they can play.

"Coach Grinch's defense isn't crazy complex," Radley-Hiles said. "It's very simple, but you have rules. Once you follow those rules, you're anticipating, you're getting ready."

Effort is non-negotiable. Riley calls the unit an "effort-based defense," and that means players flying to the ball at maximum speed and playing to the whistle. The latter part has been critical for the Sooners this season, particularly in the Big 12 championship game.

Late in the fourth quarter with the Sooners clinging to a three-point lead, Baylor quarterback Jacob Zeno hit speedy receiver Chris Platt for a 78-yard gain. Platt was sprinting behind the defense and had nobody between him and the end zone, but Oklahoma cornerback Tre Brown chased him down from behind at the 17-yard line.

The Bears ended up settling for a field goal on the drive, tying the game rather than taking the lead, and the Sooners eventually went on to win in overtime."It's a bad play defensively, but it's everything that we talk about, what we want to be as a defense," Grinch said afterward. "Find a way. If that referee doesn't throw two hands in the air, we've got a shot to get a takeaway, or to hold to three [points]."

The Sooners have done a much better job of holding teams to field goals this season. After being dead last in the nation in 2018 by allowing touchdowns on 83.3% of opponent's red zone possessions, Oklahoma has trimmed that number to 62.8%, putting them 79th nationally.

In September, Grinch emphasized the importance of effort, saying "If we're not recognized for effort at the University of Oklahoma, then I'm a con man and they're a fraud."

Mentality

From the moment their season began -- a Sept. 1 home win over Houston -- Sooners defenders have cited their mentality as a key ingredient to their success.

After that victory, which was the unit's first live test-drive under Grinch, safety Pat Fields noted that "the attitude and the mentality of everybody was completely different."

Riley says that's where their success began.

"It first starts with mentality," he said. "You can have the greatest schemes in the world on any three sides of the ball, if you don't pair it with the right mentality, you got no chance."

Grinch's message to the defense focused on a series of "one-week evaluations" rather than looking at the collective body of work.

"It's [like] golf," he said. "You birdie the first hole and triple-bogey the second, it doesn't do you a whole lot of good on the scoreboard for a round of golf. It's the same thing for our sport."

It was his way of letting the players know that, just because they had a great performance one week, it didn't mean anything the next.

"Not necessarily talking about it but being about it," defensive tackle Neville Gallimore said. "Every day, just understanding it's not about what everyone's thinking and it's not about the outside noise, it's about how we plan to operate as a group. If we're not meeting that standard, I mean, winning's great, but it's about how we're trying to do it to our standard."

That standard means that Grinch is going to critique the details. Radley-Hiles recalled a time early in the season when he got a pass deflection, but Grinch showed him where he took an incorrect step that prevented him from turning it into an interception. Players appreciate Grinch's sweating of the small stuff and attack practice with the type of attitude and effort he desires.

He understood the mental hurdles his players had to clear after difficult times in the past

"I think these kids have been through a lot," Grinch said in November. "I think sometimes I discount what we took over. Maybe there were some ghosts in there that we've gotta make sure we have an understanding of as a defensive coaching staff that we're working through."

Said linebacker DaShaun White of Grinch: "That guy's given us an identity on the defensive side of the ball. Our confidence is so much higher... Credit to him for just really practicing our tails off and giving us confidence."

'Takeaways equal victory'

Those three words are one of Grinch's oft-used expressions and they ring true. Of the top 10 teams in the country in takeaways this season, nine of them have double-digit wins. Two of them (Clemson and Ohio State) are in the playoff; LSU is only a few takeaways behind them.

It's been perhaps the biggest point of emphasis for Grinch this season. Notably, the Sooners haven't produced takeaways at quite the clip he hoped. Currently they have 11, the same number they had in 2018, which puts them near the bottom of the FBS. A five-game turnover-less streak in the middle of the season disrupted what appeared to be positive early-season progress in that department.

However, when the Sooners have taken the ball away has been critically important.

Though it didn't officially go in the box score as a turnover, defensive back Parnell Motley intercepted Iowa State quarterback Brock Purdy on a two-point conversion late in the fourth quarter of the Sooners' 42-41 win over the Cyclones.

The next week, with his team trailing by double-digits at Baylor and the season on the line, Motley stripped Bears running back Ja'Mycal Hasty to end the streak and help spark an historic comeback. Late in the game, Nik Bonitto intercepted Baylor quarterback Charlie Brewer to seal that win.

Players say Grinch yells "Takeways! Takeaways!" before, during and after every drive. And if the Sooners don't have one by halftime, he'll write the word on the whiteboard immediately.

The five-game turnover-less streak in midseason was bookended by six takeaways in the Sooners' first four games and five in their last four.

While the defense has been good overall, the turnover ups and downs are symbolic of the fact that the Oklahoma defense has been far from perfect. The 48 points it allowed to Kansas State, when the Wildcats ran wild over the Sooners, was an eye-opener. The 20 points allowed in the fourth quarter of Iowa State was disconcerting as well. And a poor first half in the November clash with Baylor -- though it was aided by offensive turnovers -- made life difficult.

Facing LSU, the nation's third-highest scoring team, the FBS leader in yards per game, will be Oklahoma's toughest challenge to date. Not only will the Sooners face Heisman Trophy winner Joe Burrow and Biletnikoff Award winner Ja'Marr Chase, they'll have to do it without two key players. Oklahoma's sack leader, defensive end Ronnie Perkins, is suspended for the game and Delarrin Turner-Yell appears unlikely to play after suffering an injury in practice.

Riley said Monday that the Sooners are "not a one-man show up front" and expressed confidence in his team's ability to compensate for the loss of Perkins. Gallimore, who has been consistently disruptive at nose guard, and Kenneth Murray, a first-team All-Big 12 selection at middle linebacker, have been key cogs to the front seven all year long.

Regardless, the Sooners wouldn't be here without the efforts of their defense. In the Big 12 championship, Riley's offense had a fourth-and-3 at the Baylor 47 with fewer than 90 seconds left in a tie game. Rather than go for the win with Jalen Hurts and CeeDee Lamb at his disposal, Riley opted to punt, trusting his defense.

Baylor played for overtime and in that period, the Sooners smothered Zeno to secure their spot in the playoff.

It was the culmination of a season-long effort to turn things around, which the Sooners did successfully.

"We wanted to be the starting point for something special," Gallimore said. "Coach Grinch gave us that opportunity."