[2] O'Brien and her twin sister were raised in Massapequa Park, New York, after moving to America from Ireland at two years old.

A Catholic, she attended St. John the Baptist Diocesan High School in West Islip.

[citation needed] She rowed at the University of Virginia, and was named the 2012 USRowing Fan's Choice Collegiate Rower of the Year.

[3] In 2018 she was selected to the women's eight and rowed bow-seat at the World Championships, obtaining the gold medal.

[6] At these Olympics, she rowed stroke seat in the women's eight, leading the USA to first place in the heat and thereby qualifying directly to the A final.