He plays Eddie Gibson, a meddlesome sergeant who's been an occasional character for three years and is best known for a disastrous attempt at fixing up his niece with Andy Sipowicz (Dennis Franz (search)). The two characters have been at odds ever since.
"Eddie is old school, a good guy who wants to do the right thing, but is just not the sharpest knife in the drawer," says O'Donohue, 58, who will become a regular on the show — replacing Esai Morales — in the March 2 episode.
O'Donohue's role will draw on years as a real-life cop.
He joined the force in 1968 and rose through the ranks to uniform lieutenant — working in Harlem and the Midtown North precincts, and later in the organized crime unit.
He spent a year as a bodyguard for Det. Robert Leuci, who inspired the 1981, Oscar-nominated movie "Prince of the City." He retired from the force in '88 — after 20 years.
Along the way, he braved shootouts, harrowing drug busts and gambling den raids.
His son, Sean, 35, became a cop too — in the same Midtown North precinct as his father.
O'Donohue recalls his scariest moment as cop came in his very first year in uniform. While working at the 46th precinct in The Bronx, he and his partner followed a trail of blood into an apartment and kicked open the door to find a burglar 7 feet away, pointing a gun at them.
"I fired two shots before the door swung shut, then kicked back the door and fired again" hitting the burglar, says O'Donohue. "He didn't die."
"I found out later that the guy had actually pulled the trigger, and the bullet had misfired. But the really scary part was when we went into the apartment and couldn't find the gun.
"We looked and looked, and I kept saying, 'I know I saw a gun.' It turned out that when the guy went down on the floor, it slid under the table."
By comparison, the actor's life is piece of cake.
Ben Stiller, who gave O'Donohue his first career break, says he initially hired the ex-cop because of his skill at using obscenities with utter believability. "No one I know curses as beautifully as John," says the star.
Only later did Stiller find out O'Donohue could act, too. "He is a really good actor and has the life experience to bring to his work that most actors wish for," Stiller says.
O'Donohue caught the acting bug while still a cop, eventually enrolling in acting classes and auditioning for Off-Off Broadway shows. Just before O'Donohue retired, Stiller discovered him at an open casting call.
"I love John," says Stiller. "We first met when I was casting the first thing I ever directed, a comedy special for Colin Quinn (search) on MTV.
"We were doing an Irish wake scene, and we needed someone to play Colin's twisted uncle. O'Donohue came in and blew us all away. Later, I found out he was a retired cop."
In 1990, O'Donohue moved to Los Angeles, where he quickly landed a role as a regular — Max the bus driver — on the short-loved "John Larroquette Show." (search) Later came guest spots on "Seinfeld," (search) "Mad About You," "King of Queens," "Everybody Loves Raymond," "ER," "Chicago Hope" and another Steven Bochco show, "Brooklyn South," where he caught the attention of the "NYPD Blue" brass.
"Aside from being a fine actor, he makes my job a lot easier," says Bill Clark, an "NYPD Blue" executive producer who is also a retired NYPD veteran.
"His knowing how to carry himself, how to wear a shield or a gun, all helps in keeping it real."