O'Brien was born to Erika and James O'Brien, and has two older brothers, Jack and Max. She attended Noble and Greenough School in Dedham, Massachusetts during her freshman year. She then joined Shattuck-Saint Mary's in Faribault, Minnesota for her sophomore year.[1][2] During her sophomore year, she recorded 37 goals and 43 assists in 56 games. During her junior year, she recorded 52 goals and 42 assists in 49 games and was named the USA Today Girls Hockey Player of the Year.[3][4]
During the 2021–22 season, in her sophomore year, she recorded 27 goals and 28 assists in 38 games. She led the nation with nine game-winning goals and ranked third in program history.[1] Following the season she was named a top-ten finalist for the Patty Kazmaier Award.[7] During the 2022–23 season, in her junior year, she recorded 19 goals and 29 assists in 41 games, and helped the Badgers win the 2023 NCAA division I tournament.[1][8]
During the 2023–24 season, in her senior year, she recorded 23 goals and 50 assists in 41 games, and helped the Badgers finish as runner-ups in the 2024 NCAA division I tournament. Her 50 assists led the NCAA, and ranked first in single-season program history. Her 73 points ranked second in the NCAA, behind teammate Kirsten Simms' 75 points. She became the eighth payer in NCAA Division I history to score 50 or more assists in a season. During the month of January 2024, she led the WCHA in points with 17, had five multi-point games, and recorded a point in every game during the month. She was subsequently named the WCHA Player of the Month.[9] During the 2024 WCHA tournament she recorded 13 points and was subsequently named to the All-Tournament team, and tournament MVP.[10] Following an outstanding season, she was named to the All-WCHA first team, USCHO Player of the Year, and a top-three finalist for both the Patty Kazmaier Award and WCHA Forward of the Year.[11][12][13][14]
On May 8, 2024, O'Brien announced she would use her COVID-19 exemption and return to Wisconsin for a fifth year.[15] On September 18, 2024, she was named co-captain for the 2024–25 season, along with Caroline Harvey.[16] On October 5, 2024, O'Brien scored one goal and two assists in a game against Boston College to become the seventh player in program history to surpass 200 career points.[17] During the first month of the season, she recorded two goals and six assists to lead the WCHA in scoring. She led the WCHA in assists (6), assists per game (3.00), total points (8) and points per game (4.00) and was subsequently named the WCHA Player of the Month for the month of September 2024.[18] During October, she again led the WCHA in points (16) and assists (11) and was named WCHA Forward of the Month for the second consecutive month.[19][20]
O'Brien represented the United States at the 2018 IIHF World Women's U18 Championship where she recorded three goals and two assists in five games and won a gold medal. During the semifinals against Canada, she scored the game-winning shootout goal to help the United States advance to the gold medal game.[21] She again represented the United States at the 2019 IIHF World Women's U18 Championship, where she recorded one goal and two assists in five games and won a silver medal.[22][23]
