Browns requested permission for OC interviews w/ Cowboys OC Bill Callahan and DAL asst Scott Linehan, per source. Jags requested Callahan.
— Adam Schefter (@AdamSchefter) January 13, 2015
Callahan had some success historically as an offensive coordinator during his time with John Gruden and Marc Trestman, but he has done a good job building and molding the Cowboys offensive line and designing their running game.
Callahan served as the offensive coordinator under John Gruden with the Oakland Raiders from 1998-2001 and the head coach from 2002-2003 after John Gruden left. After a failed stint as the Nebraska head coach, Callahan spent time with the New York Jets as their offensive line coach before being brought to the Cowboys.
Callahan has a wealth of NFL experience and has been a part of some good offensive units in his past.
UPDATE: Ryan O'Halloran had an interesting thought on Callahan:
If the Jaguars decide to interview Bill Callahan for their offensive line coach, which we're still not sure of, I personally think it would be the best bet. Some of the Cowboys coaches are free agents now that their season has ended and Callahan was not the play caller for the Cowboys.
He's a renowned offensive line guru, so if the Jaguars are moving on from George Yarno, who missed the season undergoing cancer treatment, Callahan would be an excellent replacement. The Jaguars haven't committed to the future of Yarno with the team one way or another, but it's also possible Yarno may want to retire from coaching.