O'Neil, 37, takes over the position that Eric Mangini held on Jim Tomsula's staff last season. The 49ers ranked 29th in the NFL in yards allowed last season and 18th in the league in points allowed at 24.2 points per game.
O'Neil served as the defensive coordinator in Cleveland for the last two seasons. In 2014, his unit finished 9th in scoring and 23rd in yards allowed. Last season, the Browns defense finished 29th and 27th, respectively. Given Kelly's penchant for up tempo offense, his defense is usually one to target in fantasy matchups.