Over the course of his career, O'Brien caught eleven different Cy Young Award winners: Frank Viola, Dwight Gooden, Bret Saberhagen, David Cone, Greg Maddux, Tom Glavine, John Smoltz, Pat Hentgen, Roger Clemens, Chris Carpenter and Jack McDowell. However, he only caught three of these pitchers during Cy Young seasons—Clemens, Hentgen & Maddux. During Maddux's 1994 and 1995 seasons when he posted ERA's of 1.56 and 1.63 respectively, O'Brien generally served as Maddux's personal catcher. O'Brien later testified at Clemens' perjury trial.