O'Hagan joined the New York City Fire Department in 1947 at the age of 22, and quickly rose up the ranks.
He earned a reputation as a brilliant fire officer and a tough manager, despite his initial lack of knowledge of how to work the levers of city government.
Even Chief O'Hagan, commanding a leader as he was, could not thwart a 1968 revision of the building code, drafted in large part by the real estate industry, that he thought thinned the margin of fire safety.
But they did not apply to the World Trade Center, which, being owned by another government agency, the Port Authority of New York and New Jersey, was exempt from city codes and fire inspections.
[1] O'Hagan was married to Kaye Tully, had three children (Catherine, Susan and Michael) and ten grandchildren (Clare, Molly, Cecilia, Emily, Elizabeth, John, Daniel, Maggie, Joseph, and Michael).