PROFESSIONAL
2025/26 (Liverpool FC): O'Sullivan joined Liverpool in January 2026 after spending nearly nine seasons with the North Carolina Courage. She made 13 appearances across Women's Super League and FA Cup competition, helping Liverpool reach the Women's FA Cup semifinals.

2025 (North Carolina Courage): O'Sullivan captained the Courage during the first half of the NWSL season, making 18 regular season appearances and recording two assists before transferring to Liverpool. She departed as the club's all-time appearance leader and one of the most decorated players in franchise history.

2024 (North Carolina Courage): O'Sullivan started 23 regular season matches, recording one goal and three assists while helping the Courage reach the NWSL Playoffs.

2023 (North Carolina Courage): O'Sullivan was named captain of the Courage and started 20 regular season matches she appeared in. She helped North Carolina capture the NWSL Challenge Cup while anchoring one of the league's strongest midfields.

2022 (North Carolina Courage): O'Sullivan appeared in 19 regular season matches, scoring one goal and adding two assists as the Courage won the NWSL Challenge Cup. Her performances earned league-wide recognition as one of the NWSL's most consistent defensive midfielders.

2021 (North Carolina Courage): O'Sullivan started all 23 matches she appeared in during the NWSL regular season, continuing her role as the heartbeat of North Carolina's midfield.

2017-20 (North Carolina Courage): O'Sullivan became a cornerstone of one of the NWSL's greatest dynasties after joining the Courage in 2017. She helped the club win back-to-back NWSL Championships in 2018 and 2019, three consecutive NWSL Shields (2017-19) and established herself as one of the league's elite midfielders. She was voted the Courage's MVP by her teammates in both 2018 and 2019.

2016 (Houston Dash): O'Sullivan made her NWSL debut with the Houston Dash after successful professional spells in Ireland and Scotland.

2013-15 (Glasgow City): O'Sullivan helped Glasgow City dominate Scottish football, winning three consecutive Scottish Women's Premier League titles while earning the 2014 SWPL Players' Player of the Year award. She also competed in the UEFA Women's Champions League.

2011-13 (Cork City/Peamount United): O'Sullivan began her senior career in Ireland with Cork City and Peamount United, competing in the Women's National League and UEFA Women's Champions League before moving to Scotland.

INTERNATIONAL
O'Sullivan has been a fixture for the Republic of Ireland for more than a decade. She made her senior debut in 2011 at age 17 and has earned more than 120 caps for her country. She helped Ireland qualify for its first-ever FIFA Women's World Cup in 2023 and has twice been named FAI Senior Women's International Player of the Year.