He is married to Liz Truss, who served as Prime Minister of the United Kingdom from September to October 2022.

His mother Susan (née Dunn) was a nurse, and his father John O'Leary was a college lecturer who trained as a solicitor and worked for the legal firm of the Liverpool lawyer Rex Makin.

[3] Whilst at university, he was acquainted with Thérèse Coffey, who was later Liz Truss's deputy prime minister.

[6] O'Leary has stood unsuccessfully for the Conservative Party in local elections to Greenwich London Borough Council on several occasions.

[3][8][9] His most recent candidacy to date was at the 2006 Greenwich election, this time in the Charlton ward, when O'Leary and the other opposition parties finished a considerable margin behind the winning Labour candidates.