Michael Gaston, who plays Parish, appeared 5 times (playing different roles each time) in Law & Order (1990) and 4 times (playing different roles each time) in Law & Order: Special Victims Unit (1999). All three shows happen in the same universe and they were all created by Dick Wolf.
Kevin J. O'Connor plays the new commander, Fischer. Kevin was in The Mob Doctor (2012) with Jesse Lee Soffer who plays Jay Halstead. In addition, both shows are taking place in Chicago.
Michael Gaston (Parish) returns in season 10 playing a different character, Chief Patrick O'Neal. Erik LaRay Harvey (Devon Tucker) also returns in season 10 playing Kevin's father, Lew Atwater.