O’Connor, who also starred in Florence Foster Jenkins, replaces Julian Baring, while Bailey replaces Victoria Hamilton.
It comes as production on season three is in full swing ahead of a 2019 launch.
O’Connor, said, “I am thrilled to be joining The Firm for the next instalment of The Crown. Seasons 3 and 4 will follow some of the most turbulent events in the Prince of Wales’s life and our national story and I’m excited to be bringing to life the man in the midst of it all. I’m very aware it’s a formidably talented family to be joining, but reliably informed I have the ears for the part and will fit right in.”
Bailey added, “Wonderful to be joining The Crown. It’s a brilliant show and we have a tough act to follow but what a gift to be playing the fascinating and greatly loved Queen Mother. Thrilled to be on board and working with such a top-notch team.”
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