O'Sullivan was born in Harrow, London and raised in Ickenham.

Her father is a construction contracts manager and her mother works in the NHS.

She started dance at age two, and performed with London Children's Ballet.

Her first principal role was Alice in Alice's Adventures in Wonderland, and she went on to portray other lead roles such as Juliet in Romeo and Juliet, Princess Aurora in The Sleeping Beauty and Swanida in Coppelia.

[1][5] In May 2021, the Royal Ballet announced that O'Sullivan's promotion to principal dancer will take effect in September 2021.