“I’m not doing any more”, she revealed.
“O’Brien is a thoroughly despicable human being – that was great to play.”
A spokeswoman for the show confirmed O’Brien will not be featured in the fourth series.
The hit drama already sees the exit of other key cast members in Season 3.
Creator Julian Fellowes recently told The New York Times: “When an actor playing a servant wants to leave, there isn’t really a problem — (he or she just) gets another job. … With members of the family, once they’re not prepared to come back for any episodes at all, then it means death.”