[1] O'Neil was previously the branch and national secretary of the Textile Clothing and Footwear Union of Australia (TCFUA).

In 2004, she told union members to consider cutting ties with the Australian Labor Party (ALP) over tariff cuts in the clothing, footwear and textiles industries.

[1] O'Neil spoke in favour of scrapping boat turn-backs in 2015 when a delegate at the Labor party conference.

[citation needed] In 2018, O'Neil was elected president of the ACTU.

O'Neil's father served in the army, before serving as a public servant and briefly as an Australian Football League (AFL) administrator.