After playing football for the University of California, Davis, J.T. O'Sullivan was drafted by the Green Bay Packers in round six of the 2002 NFL Draft. He was the 186th pick overall. In 2004, he made his NFL debut with the Packers. In 2007, he went to the Detroit Lions and in 2008, he signed a deal with the San Francisco 49ers. He's presently third on the 49er depth chart.

O'Sullivan played in only one game with the Pack, accumulating no passing stats. He ran two plays and took a knee on both, losing a total of two yards. In 2007, he appeared in five games with Detroit, throwing 26 passes and completing 13 for a 50.0-percent completion rate. He tossed one touchdown and two picks, earning a rating of 48.2. With Detroit, he fumbled three times, losing one.

The six-foot-two, 227-pound quarterback is the minimal size to be effective as a professional quarterback. Height is a problem for O'Sullivan, who could use another two inches to see above defenders. Still, the QB is a good athlete with decent strength and quickness. He ably moves around in the pocket, giving himself extra time to find and hit receivers. His lack of experience has left him unrefined and raw. He's not very good at reading or checking off on defenses, and he often tries to force the ball into tight situations when he shouldn't. At this point, O'Sullivan looks to be a backup for the rest of his career.