Complex sentence: Callahan was fired from Nebraska in 2007 by Tom Osborne, who had retured as athletic director halfway through the season, after going 5-7, the program's only 2nd losing season since 1961 (both under Callahan).
Split into simpler sentences: Callahan was fired from Nebraska in 2007 by Tom Osborne, who had returned as athletic director halfway through the season, after going 5-7. This was only the second time the program had a losing season since 1961 (both under Callahan).