[8] O'Neill began an arts degree, but withdrew when her younger sister, Helen, was diagnosed with acute myeloid leukemia.

[6] When interviewed in 2023, O'Neill indicated that family and community life gave her a strong sense of social justice, which propelled her first into a teaching a career, then politics.

[11][8] It is reported that, having established her home, and pregnant with their third child, O'Neill heard Paul Keating's 1996 speech on creating a modern Australia; and then decided to join the Labor Party.

[21][22] O'Neill joined the Senate on the opposition benches of the 44th Parliament of Australia serving on the Joint Standing Committee on Foreign Affairs, Defence and Trade.

[26] Her strong opposition to Matthias Corman's proposal to reduce corporate tax in 2018 led to her being listed amongst "Women Who Are Absolutely Done Listening To Men.

[28] Using this position she pressured Australian Securities & Investments Commission to investigate AMP Limited for unfair contracts with financial planners.

[29] She also accused AMP Limited, EY and KPMG of allowing sexual harassment and bullying in their organisations, and challenged a member of the Fair Work Commission for displaying sexualised waifus in a government office.

[35] During this Parliament, O'Neill held senior party positions, including Deputy Co-Chair of the ALP's National Policy Forum.

[8] In the run up to the 2022 Australian federal election, O'Neill refused to relinquish the top position for the Labor Senate ticket to Kristina Keneally, leading to a bitter internal contest, in which she prevailed.

[36][37] Prime Minister Anthony Albanese appointed her to the committees for Treaties, for Corporations and Financial Services and for Foreign Affairs, Defence and Trade – which she chairs.

[18][38] O'Neill has asserted Labor's commitment to introducing new laws, in 47th Parliament of Australia, allowing faith-based schools to select staff of that faith.

[40] In this role she focused the committee on "uncovering the full extent of failure points" amongst the Big Four accounting firms, particularly their "lack of accountability" to "ensure the national interest prevails.”[4][40] Having uncovered the 144-page cache of internal PwC emails, O'Neill played a leading part in exposing the PwC tax scandal, revealing that 30 partners and staff were involved.

[41] O'Neill is reported to carry two items in her handbag: a set of pink rosary beads and a copy of the Universal Declaration of Human Rights.