College during Feb. 4, 2024 game O'Brien played five seasons at the
University of Wisconsin Badgers from 2020 to 2025, winning three
NCAA national championships and finishing as the program's all-time leading scorer with 274 points. She began her
collegiate hockey career during the
2020–21 season. During her freshman year, she recorded two goals and eight assists in 21 games, and helped the Badgers win the
2021 NCAA Division I tournament. 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. Following the season she was named a top-ten finalist for the
Patty Kazmaier Award. 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. 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 player 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. During the
2024 WCHA tournament she recorded 13 points and was subsequently named to the All-Tournament team, and tournament MVP. 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. On May 8, 2024, O'Brien announced she would use her
COVID-19 exemption and return to Wisconsin for a fifth year. On September 18, 2024, she was named co-captain for the
2024–25 season, along with
Caroline Harvey. 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. 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. 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. During the
2024–25 season, as a graduate student, she recorded 26 goals and 62 assists in 41 games. She led the nation in assists, points, and points per game (2.20). She recorded 17 goals and 35 assists in 28 conference games, and was named WCHA scoring champion. Following the season she was named the WCHA Player of the Year and Forward of the Year. She also won the 2025 Patty Kazmaier Award. On March 1, 2025, during the
2025 WCHA women's ice hockey tournament against
Bemidji State, O'Brien recorded one goal and five assists, to become Wisconsin's all-time leading scorer, surpassing
Hilary Knight's record of 262 career points. One week later, during the WCHA championship game against
Minnesota, she recorded one goal and two assists, to become Wisconsin's all-time leading scorer, male or female, surpassing
Mike Eaves' record of 267 career points. She finished her career as Wisconsin's all-time leader in points (274) and assists (177). Her 274 points rank eighth in NCAA history.
Professional New York Sirens, 2025-present On June 24, 2025, O'Brien was drafted third overall by the
New York Sirens in the
2025 PWHL Draft. The Sirens acquired the pick at the draft in a trade that sent
Ella Shelton to the
Toronto Sceptres. During the preseason, O'Brien led all Sirens rookies with three assists in two games while centering a line with reigning
PWHL Rookie of the Year Sarah Fillier. On November 18, 2025, she signed a one-year contract with the Sirens. After recording two assists in her first seven games of the
2025-26 PWHL season, O'Brien broke through on December 28, 2025, scoring her first
hat trick (three goals) against the
Seattle Torrent during a
PWHL Takeover Tour game in
Dallas. She scored one goal in each period, including the game-winning
power play goal with 3:20 remaining in the third period, as the Sirens rallied from a 3–2 deficit to win 4–3. The game was played at
American Airlines Center in downtown Dallas in front of 8,514 fans, marking the southernmost PWHL game in league history. Following the hat trick, O'Brien recorded her third assist of the season on January 2, 2026, in a 4–3 victory over the
Montreal Victoire, giving her five points in her last four games. ==International play==