O'Hare (Take Me Out, Assassins), also known for "True Blood," joins a cast that includes Tony Award winner Patti LuPone, Angela Bassett and Kathy Bates, as well as cast members from previous seasons, including Jessica Lange, Lily Rabe and Sarah Paulson.
O'Hare is also part of Murphy's film adaptation of "The Normal Heart," which will air on HBO in 2014.
"American Horror Story: Coven" will begin filming later this summer in New Orleans.