She also played for Peamount United in the County Dublin club's UEFA Women's Champions League campaigns in 2011 and 2012.

The youngest of nine children, her brother John Paul played soccer for Ireland at youth international level.

[3][5] She played for a boys' team, Nufarm Athletic, until age 11 when she had to leave due to rules against mixed-sex football.

[8] On 7 August 2011, she scored a goal in the FAI Women's Cup final, as Wilton United were defeated 3–1 by St Catherine's.

[14] In July 2012, O'Sullivan re-joined Peamount United and made three appearances in the Champions League, scoring a goal in a 4–0 victory against Cardiff Metropolitan.

[23][24] O'Sullivan made nine appearances in the 2014–15 UEFA Women's Champions League campaign and scored four goals, including a brace in a 4–0 victory against Zhytlobud-1 Kharkiv on 14 August 2014.

When Randy Waldrum, who had brought her to the club, was sacked, interim coach Omar Morales restricted her to minimal playing time.

[2] The Courage finished in first place during the regular season with a 16–7–1 record, winning the NWSL Shield and advanced to the Playoffs.

[42] After defeating Reign FC 4–1 in the semi-finals,[43] O'Sullivan helped lead the Courage win their second NWSL Championship shutting out the Chicago Red Stars 4–0 in the final.

[44] Teammate Sam Mewis said O'Sullivan was 'the "glue" that holds the team together, winning tackles and spraying the ball wide from a deeper position.

She is a tremendous footballer and passer of the ball and she breaks up the opponent's playmaking ability with an intuitive reading of the game.

[47] O'Sullivan played every minute of the preseason 2022 NWSL Challenge Cup, in which the Courage defeated the Washington Spirit 2–1 in the final.

[4] For the 2019–20 W-League season, O'Sullivan was loaned once again to an Australian club, joining her North Carolina Courage teammates Lynn Williams and Kristen Hamilton at Western Sydney Wanderers as a guest player.

[60] Their fourth place finish earned the team a berth to the semi-finals where they were defeated 5–1 by eventual champions, Melbourne City.

[61] In September 2020, O'Sullivan moved on loan to English FA WSL club Brighton & Hove Albion through 31 December 2020.

Travelling back and forth from the United States would force her to undergo three separate two-week periods of quarantine within nine weeks.

[63] She made her debut for Brighton as a half-time substitute for fellow Corkonian Megan Connolly in a 0–0 draw at Manchester City on 13 September 2020.

[2] O'Sullivan was part of the under-17 team that finished in second place at the 2010 UEFA Women's Under-17 Championship, after losing on penalties in the final against Spain.

At the 2010 FIFA U-17 Women's World Cup in Trinidad and Tobago, she scored in the quarter-final defeat against eventual runners-up Japan, after helping Ireland finish top of a group that included Brazil, Canada and Ghana.

[65] On 17 September 2011, O'Sullivan made her senior debut and scored both goals in a 2–0 UEFA Women's Euro 2013 qualifying Group 4 win over Wales in Newport.

[66] She headed Ireland into the lead over Scotland in their Euro 2013 qualifier at Tynecastle Stadium in April 2012, but the Scots staged a late comeback to win 2–1.

[67] O'Sullivan continued to be selected by national team coach Susan Ronan and participated in Ireland's failed 2015 FIFA Women's World Cup qualification campaign.

[69] She remained an important national team player under Ronan's successor Colin Bell, displaying good form in the unsuccessful 2019 FIFA Women's World Cup qualifying series.

"[3] In December 2019, she was included in The Guardian's The 100 Best Female Footballers In The World and was described as "a combative presence in the Courage midfield [and] the heartbeat of Paul Riley's successful side.