In 1959 she received a diploma in tropical medicine and hygiene, and for the next 14 years she worked in mission hospitals in west Africa.
Alongside her colleagues, O'Brien opened the St Charles Borromeo hospital in Onitsha, Nigeria in 1964, where she was the medical superintendent.
O'Brien and her fellow aid workers worked in dangerous conditions, witnessing the famine which then devastated the local area.
[1][3] This allowed O'Brien to travel to England for further training in gynaecology, obtaining Membership of the Royal College of Obstetricians and Gynaecologists in 1971.
She worked there until 1999, treating and operating on thousands of women and facilitating the training hundreds of doctors at postgraduate level.