O'Hara completed an undergraduate degree in history at Oxford University between 1993 and 1996, before completing a M.Sc. in Economic and Social History in 1996–1997, during which time he won the Eubule Thelwall Prize for history and the Gladstone Prize for history and politics.^[1] Following graduation he briefly worked as a schoolteacher. From 1999 he studied for a PhD in history at University College London under the supervision of Professor Kathleen Burk; he graduated in 2002. In 2001 he was appointed lecturer in economic history at the University of Bristol, where he spent a year before moving to New College, Oxford as lecturer in modern history. In 2005 he moved to become a lecturer at Oxford Brookes, and was promoted to senior lecturer a year later. He became reader in the history of public policy in 2010 and professor of modern and contemporary history in 2013.^[2]

O'Hara writes on the subject of contemporary politics in the United Kingdom for The Guardian, The Independent, The New European and GQ magazine.^[3][4][5][6]

In 2019 he was an advisor to the short lived centrist political party Change UK.^[7] In a paper titled: 'What next for the Independent Group? Here's a winning manifesto', O'Hara argued for limited cuts to local government and more devolution coupled with greater focus on cooperative systems for managing public services, rather than full nationalisation. Alongside a 'Third Way' politics in the tradition of New Labour, O'Hara favoured a pro-EU position and the return of full living grants for poorer students to pursue academic study.^[8] The party was dissolved later that year after it failed to win seats in local elections, the 2019 United Kingdom general election or the 2019 EU Parliament elections.

• The Politics of Water in Post-War Britain (Basingstoke: Palgrave, 2017).^[9]
• Governing Post-War Britain: The Paradoxes of Progress (Basingstoke: Palgrave, 2012).^[10]
• Britain and the Sea since 1600 (Basingstoke: Palgrave, 2010).^[11]
• Statistics and the Public Sphere: Numbers and the People in Modern Britain, c. 1800—2000 (edited with Tom Crook, London and New York: Routledge, 2011)^[12]
• From Dreams to Disillusionment: Economic and Social Planning in 1960s Britain (Basingstoke: Palgrave, 2007)^[13]