O'Sullivan was born and raised in Little Rock, Arkansas to an accountant mother and a father who worked in health insurance.
[2][4] O'Sullivan always wanted to be an actor and, supported by her cinephile parents, she performed in local children's theatre plays from age five.
[6] Inspired by Greta Gerwig, who directed Lady Bird, O'Sullivan wrote Saint Frances based on the experiences she had while working as a nanny to support her struggling acting career in her twenties and an abortion she had in her early thirties.
She started work on the script in January 2018, already intending the film to be directed by partner Alex Thompson.
She wrote the parts for the lesbian parents in the film for actors Lily Mojekwu and Charin Alvarez.