Director James Wan, rising high on the success of "Furious 7", returns as director and producer along with Peter Safran and Rob Cowan, said The Hollywood Reporter.
O'Connor, 48, will play the mother of a girl, who is experiencing a haunting and desperate for help.
Vera Farmiga and Patrick Wilson are reprising their roles of real-life paranormal investigating couple Lorraine and Ed Warren.
"The Conjuring 2" is due to begin production later in September.
Wan wrote the screenplay with Chad and Carey Hayes, with revisions by David Leslie Johnson.