She began her career as a journalist in the 1970s. Since then, she has held senior positions with The Irish Press and the Sunday Tribune, as well as serving as a political columnist at The Sunday Times and as the Political Editor of The Sunday Business Post. In 1991 she made an extended appearance on the British television discussion programme After Dark, alongside among others Patrick Cosgrave, J. P. Donleavy, David Norris and Francis Stuart. In 1998, she became the editor of Magill magazine. She resigned in September 1999 when the magazine's sister publication, In Dublin, was banned by the Censorship of Publications Appeal Board for advertising brothels and prostitution services. O'Reilly was also a broadcaster on Raidió Teilifís Éireann and Today FM. In the course of her journalistic career, she won two awards: Journalist of the Year and Woman Journalist of the Year.
Transparency and accountability of Irish public bodies
In a speech delivered in Dublin on 20 June 2006 to the Institute of Public Administration, O'Reilly criticised "some service providers, both public and private" for retreating from dealing personally with the public through the use of call centres and the Internet. She mentioned the Irish Revenue Commissioners in this context, pointing out that a significant proportion of the clients of these bodies "do not have access to the web" and therefore the level of personal contact is inadequate as a consequence. She also believed that public access to information under the Freedom of Information Act had been "excessively curtailed", often in order to protect sectional interests, such as the performance of schools. She advised that the Act should be extended to include a number of public bodies previously exempted from the law, including the Garda Síochána, the Central Bank of Ireland and the National Asset Management Agency and that fees charged were a further inhibitor. O'Reilly is the author of three books: Candidate: The Truth Behind the Presidential Campaign, about President of IrelandMary Robinson; Masterminds of the Right about politicised Catholic fundamentalism in Ireland; and the controversial biography, Veronica Guerin.
European Ombudsman
O'Reilly was appointed European Ombudsman by the European Parliament in 2013, and re-appointed in 2014 and 2019. Her current term expires in 2024.