In 2003 O'Hare played a doctor in "21 Grams". In 2004, he played Charles J. Guiteau in the Broadway production of Stephen Sondheim's "Assassins", for which he was nominated for the Best Performance by a Featured Actor in a Musical Tony Award. He lost to co-star Michael Cerveris who played John Wilkes Booth. Before appearing in those shows, he appeared on Broadway in the 1998 revival of "Cabaret", in which he played Ernst Ludwig onstage and the clarinet in the show's orchestra, the Kit Kat Band. O'Hare was featured in the Hallmark Hall of Fame presentation of "Saint Maybe". He has appeared as a guest star on several episodes of "Law & Order" and its spin-offs, ' and '.