One thing to start out with:
Under head coach Bob Stoops, Oklahoma won 11 out of 13 games against between 1999 and 2011. The Sooners usually featured at least All American on each one line (Tommie Harris, Gerald McCoy) and either a Heisman Trophy candidate or winner (Adrian Peterson). No matter what, A&M usually found itself with less talent than what the Sooners had and it always seemed to show up on the scoreboard.
This year, it's A&M that has the Heisman Trophy winner and multiple All Americans in its two lines. The talent differential that the Sooners have always counted on has swung in favor of A&M. Contrary to what some Oklahoma people think, this is far from the A&M team that they faced in past years.
Now, on to the matchups:
When Bob and Mike Stoops arrived at Oklahoma in 1999, they installed a 4-3 defense with a cover two shell (that is, two high safeties). Right before the snap, they would rotate one of the safeties down in the box much like Mike Stoops did at Kansas State with Jerrod Cooper and they used former Dallas Cowboy Roy Williams in the same manner in their early years at Oklahoma.
In addition, the Sooners recruited multiple people that were 6 feet plus and around 200 pounds. They rotated them throughout the secondary and even weakside linebacker because they played a lot of two deep zones. The safeties played deep halves and kept everything in front of them. They needed big corners who could come up in run support and who could bump receivers in the shorts zones.
That philosophy got severe tests in BCS bowl game losses to USC, Boise State, West Virginia, and Florida because bigger back seven defenders got put into space and had issues both covering and tackling. In addition, the Big 12 transitioned from being a power running conference to a conference with teams that ran the spread. The Sooners started recruiting smaller people in the back seven and also transitioned to blitzing more and using multiple fronts. They played more man coverages and tried to force turnovers with pressure. Sometimes, the Sooners got to the quarterback (second in the Big 12) but other times (particularly Oklahoma State and Texas Tech last season) they just gave up big plays (fifth in the Big 12 in 2011).
With the return of Mike Stoops as defensive coordinator, Oklahoma has changed up its defense this season. They play a lot more two deep safeties, stay primarily with a four man front, and don't blitz as much. More importantly, they essentially play five and six people in the box, even when the offense equals or outnumbers them with five offensive linemen and a back.
As a result, Oklahoma doesn't give up a lot of big plays in the passing game (second in the Big 12 in passing plays over 10 yards). Their secondary (led by safety Tony Jefferson) is very good and linebackers Tom Wort and Corey Nelson are athletic. However, opponents have found other ways to move the ball on them, particularly in the running game (the Sooners rank eighth in the Big 12 in run defense) because they are typically outnumbered in the box. In addition, their linebackers are not very big (6 foot 1 or less, even the backups) and their secondary (with the exception of Jefferson) lacks size to come up in run support or make big hits. They don't allow many big plays but they have issues stopping the run, they don't force many turnovers (ninth in the Big 12 in turnovers forced), and they don't get many sacks (24 on the season). Even though they try to play a bend but don't break style, they are eighth in the Big 12 in red zone touchdown percentage so they still give up points.
In contrast, Texas A&M has Heisman Trophy winning quarterback Johnny Manziel. The Sooners haven't faced anyone like him all season that can generate offense BOTH running and passing. It's almost a given that the Aggies are going to be able to run the ball on the Sooners by alignment because they may not keep enough defenders in the box as to not give up the big play. Also, there's not a matchup that doesn't favor A&M across the line of scrimmage. None of Oklahoma's linemen are first step guys who make plays in the backfield and with the suspension of defensive tackle Stacy McGee, the Sooners lack size as well. DE David King (286 pounds) will move inside for this game which means that A&M will have a significant advantage in size and experience in the interior even before you consider highly athletic tackles Luke Joeckel and Jake Matthews.
Thus, even if the Sooners don't allow the Aggies to generate a lot of big plays, A&M can still move the ball into the red zone and should expect to score touchdowns there due to advantage that they have in the line.
Manziel himself translates into the hardest possible matchup for a defense…he's a quarterback who can make big plays and who's gotten where he doesn't turn the ball over (just three of them in the Aggies' final five games). A&M's offensive line is also one of the best pass protection units in the country and gives Manziel time to move around and either extend passing plays or take off. In addition, playing people off the ball would (on the surface) translate into the ability to keep Manziel's running in check. However, spying him simply gives him a significant advantage over a defender because he has more space to work with.
In the passing game, the way that the Sooners have recruited defensive backs means that they really don't have anyone to match up with Mike Evans. Both Aaron Colvin and Demontre Hurst give up five plus inches and forty plus pounds to Evans who can win one one matchups either by design or simply by being a better athlete. Ryan Swope has caught 16 passes in his last two games versus the Sooners for 285 yards and two touchdowns. He's been able to get open against Oklahoma's slot defenders because the Sooners don't match up well versus slots (Oklahoma State's Josh Stewart hit them up for 11 catches and 150 yards in November) as their lack of a pass rush allows quarterbacks plenty of time to check down to them. In addition, their propensity to play zone means that slots can work to the gaps against linebackers and safeties that are usually coverage liabilities.
Overall, the current Sooners don't play defense like the Sooners of old. Even though they have recruited five star defenders such as R.J. Washington and Jamarkus McFarland, they can't stop the run or rush the passer very well. The teams that have had the greatest success slowing Manziel down were LSU and Florida and both of those teams played man in the secondary and had fast defensive ends and Oklahoma offers neither of those things. In addition, the Aggies have elevated running backs coach Clarence McKinney to offensive coordinator which should translate to an increased emphasis on the running game. The Sooners gave up an average of 43 points a game to the best offenses on their schedule (Baylor, West Virginia, and Oklahoma State) and A&M is comparable to them in productivity. In summary, it's going to be very hard for Oklahoma to win this game unless they do things that they haven't done all year.