Mike Pettine's right-hand man for the past half-decade, O'Neil oversaw defenses that finished 23rd and 27th in total yards the past two seasons. Last year's performance was especially disappointing, as the Browns finished 30th in run defense amid rumbling that the players were frustrated with O'Neil's scheme.
Assuming the defensive controls in San Francisco would be a challenge for even the most seasoned defensive coaches.
The flip side to Kelly's uptempo offense is a defense that spends an inordinate amount of time on the field. Kelly's Eagles defenses finished first, second and first in most plays faced over the past three seasons.
O'Neil has his work cut out for him if he's going to return San Francisco's defense to the heights of the Jim Harbaugh-Vic Fangio era.