The Reds have been in turmoil since they moved into Great American Ball Park in 2003. General manager Jim Bowden and manager Bob Boone were fired midway through the inaugural season, with Kullman helping to fill in until O'Brien was chosen after the season.
The Reds have had five consecutive losing seasons, their longest such streak in 50 years.
O'Brien's top goal was to rebuild a farm system that had failed to produce pitchers during Bowden's tenure. During his two years in Cincinnati, O'Brien restructured the minor league system and imposed pitch limits to end a trend of prospects getting hurt.
Reds pitchers and catchers report to spring training in Sarasota, Fla., on Feb. 16.