[3] O'Casey is from Finsbury Park, the eldest of three daughters of hospitality and retail workers.
[4] She is of Irish and a quarter Jewish descent; the playwright Seán O'Casey was her great-grandfather.
[7][8] After appearing in the music video for "Holy Show" by the Pillow Queens in 2020 and the Druid Theatre Company production of The Seagull in 2021, O'Casey made her television debut in the 1960s-set BBC One drama Ridley Road, starring as the lead character Vivien Epstein.
This was followed by a supporting role as Emilie in the Starz adaptation of Dangerous Liaisons the next year.
In 2023, O'Casey made her feature film debut in Thaddeus O'Sullivan's comedy-drama The Miracle Club and Lisa Mulcahy's period drama Lies We Tell with David Wilmot.