Which top-10 teams has the biggest holes?

At the College Football Roundtable, we ask each member of the coverage staff for their opinion about a topic in the sport.
Today’s question: Of the teams in our early preseason top 10, which one do you think has the biggest hole(s) to fill entering spring practice?

Olin Buchanan’s answer:
Last season, Oklahoma averaged 4.8 yards per rush to rank 20th in the country. Even more impressive, the Sooners ranked third in sacks allowed and the teams ahead of them – Air Force and Navy – attempted less than half as many passes as the Sooners did. Those statistics illustrate the excellence of Oklahoma’s offensive line. But four starters from the unit are gone, and three of them – Duke Robinson, Phil Loadholt and Jon Cooper – received All-Big 12 recognition. Consequently, an area that was perhaps the strongest part of the team last season figures to be substantially weakened.