He has been vice president and executive editor of Radio Free Europe/Radio Liberty; editor of the Australian monthly magazine, Quadrant; associate editor of the London Times (1986-1987); editorial page editor of the New York Post (1984-1986); assistant editor for the Daily Telegraph (1972-1979 and 1983-1984); London Correspondent for Radio Telefis Eireann; Director of Studies at the Heritage Foundation (1979-83), where he was editor of Policy Review; a senior fellow at the Nixon Center (2003-2005); a senior fellow at the Hudson Institute (2006 to 2009); editor of The National Interest (2003 to 2005); and editor-in-chief of United Press International (2001 to 2003). From 1998 to 2001 he was an editorial consultant to Hollinger International Inc. and a leading member of the journalistic team that created the National Post, Canada’s first national newspaper.

Born in Liverpool, he was educated at St Mary's College, Crosby, and received his higher education at the University of London. He stood unsuccessfully as a Conservative candidate in the 1970 British general election.

He is the founder and co-chairman of the New Atlantic Initiative, an international organization dedicated to reinvigorating and expanding the Atlantic community of democracies. The organization was created at the Congress of Prague in May 1996 by Václav Havel and Margaret Thatcher.

O’Sullivan has published articles in Encounter, Commentary, New York Times, Washington Post, Policy Review, Times Literary Supplement, American Spectator, The Spectator, American Conservative, Quadrant, The Hibernian and other journals, and he is the author of the book, The President, the Pope, and the Prime Minister: Three Who Changed the World.