Oklahoma has a possible Heisman Trophy candidate at quarterback, back-to-back Big 12 titles to defend and a top 5 ranking in the preseason for the first time since 2004.

Its defense also has a very bad taste in its mouth.

The fourth-ranked Sooners, coming off a lopsided loss in last year's Fiesta Bowl, will look to begin to answer some questions defensively when they open their season at home Saturday against Chattanooga.

Oklahoma's 2007 season was largely a success after freshman Sam Bradford finished the year with the highest quarterback rating in the country and the team captured its fifth Big 12 championship in the last eight years with a 38-17 win over Missouri in the conference title game.

The Fiesta Bowl was a different story, however, as the Sooners gave up a season-high 525 yards in a 48-28 loss to West Virginia, finishing at 11-3. After the Oklahoma defense had ranked ninth in the nation with 18.2 points allowed per game before facing the Mountaineers, West Virginia broke off four touchdown plays of at least 30 yards.

The year before in the same bowl game, Oklahoma lost 43-42 to Boise State, but Sooner coach Bob Stoops said his team's mindset has not changed heading into the 2008 campaign.

"Have I noticed any different motivation? Not really," Stoops said. "I think I've seen our guys come in virtually every year excited to play and realizing that they've got something to prove every year. Even when you've won a Big 12 championship the year before or back-to-back, it means nothing now.

"Everybody's after the same thing once again and it's your job to go try and earn it," Stoops said.

The Sooners will try to do so behind a revamped defensive unit that has to make up for the loss of linebacker and leading tackler Curtis Lofton, who left school early and was chosen in the second round of the NFL draft. Cornerback Reggie Smith, another early departure, was selected in the third round.

Oklahoma can still rely on defensive end Auston English (9 1/2 sacks last year) and linebacker Ryan Reynolds (60 tackles), but will have to fill in some other spots with relative unknowns. Keenan Clayton moved from safety to linebacker, while redshirt freshman Austin Box is battling back from knee surgery.

"It's just an experience issue across the board outside of Ryan at linebacker," defensive coordinator Brent Venables said. "I've been really pleased, and the curve is a big curve, but we'll be there. I really believe that. Whether or not there's initial growing pains, I'm certainly not counting on it but I have a perspective on it too."

Things should be relatively painless on offense, where Bradford - elected a team captain as a sophomore - will lead the way after throwing for 3,121 yards and 36 touchdowns along with eight interceptions in his first season. He has his leader in both receiving yards and receiving touchdowns back, as Juaquin Iglesias had 907 yards and Jermaine Gresham snagged 11 touchdowns.

Bradford's 176.5 passer rating in 2007 was higher even than Florida's Tim Tebow, who won the Heisman Trophy with help from his 895 rushing yards. Bradford usually sticks to throwing the ball, but Oklahoma's ground attack should be fine in the hands of sophomore DeMarco Murray, who was third on the team in carries last year, but still managed 764 yards, 13 touchdowns and an average of 6.0 yards per carry.

Slideshow   The Week in Sports Pictures The Giants on top of the football world, getting ready for the London Olympics and more. more photos

Murray had five rushing touchdowns and Bradford went 21-of-23 with three scores in last season's opener, a 79-10 win over North Texas that saw Oklahoma leading 49-0 at the half.

This year's game could be similarly lopsided considering that Chattanooga lost eight defensive starters off a team that went 2-9 last year, with all but one game played against FCS opponents. Oklahoma last played an FCS opponent in 1999, beating Indiana State 49-0.

The Mocs have never played a Big 12 team, and they're 11-36 on the road since the beginning of the 2000 season. They lost their leading rusher from last year, Bryan Fitzgerald, to a knee injury and have a new quarterback in junior college transfer Jare Gault.

Regardless of the opponent's merit, Oklahoma is simply eager to get started, especially after the way last season ended.

"They just get tired of going up against each other every day and now we finally get to face another team here at home this weekend," Stoops said. "Hopefully we can start the season in a positive, sharp way. Eliminate penalties and turnovers, and just play smart."