In 1987, O'Neill met Labour Party Politician Ken Livingstone, and subsequently became his main advisor on Irish issues. In the 1990s, he met and married Kate Hudson, who later became the Chair of the Campaign for Nuclear Disarmament. In 2000, he was appointed as Mayor of London Ken Livingstone's deputy chief of staff, responsible for transport, trade union relations, public affairs and editing The Londoner newsletter. He launched a major celebration of St Patrick's Day in London in 2002, took the lead on congestion charging, and developed links with Hugo Chavez, exchanging transport expertise for cheap fuel.

O'Neill's role in Socialist Action was widely reported in the press in advance of the London mayoral election, 2008, with the Sunday Times describing allegations that the group remained active as suggesting "Marxist infiltration". A former member of the organisation claimed that O'Neill had operated under the pseudonym "Lark".

Following O'Neill's death, at the age of 55, Gerry Adams described him as a "very dedicated and energetic supporter of Irish reunification, of the peace process and of justice and equality campaigns", the Foreign Minister of Venezuela called him "an intimate friend, a companion in the fight for equality and justice". Ma Hui, deputy head of the international department of the Central Committee of the Communist Party of China referred to his efforts "to promote and encourage better understanding and mutually beneficial cooperation between Beijing and London". Cuba's UK Ambassador said O'Neill "will forever be remembered as an Example of dedication to the anti-imperialist struggle". Member of Parliament John McDonnell stated that "Redmond's contribution to our movement will be remembered with immense respect." On 24 October 2009 the demonstration in Trafalgar Square against Britain's involvement in Afghanistan held a minute's silence marking O'Neill’s death.