The drama, to be directed by Murphy, will begin shooting next month after Murphy and Falchuk wrap production on "Glee."
The plot of the FX pilot remains under wraps, but in a breakdown for "American Horror Story," O'Hare is referred to as "Larry the burn guy."
O'Hare was most recently seen on Broadway in Elling. In 2003 he won the Tony Award for his performance in Take Me Out. He has also starred in Inherit the Wind, Sweet Charity, Assassins, Major Barbara and Cabaret.
The pilot is set to launch on FX in the fall.