O'Brien founded O'Brien Mobile Power Rental Co. in 1968 and it eventually boasted the world's largest fleet of mobile-power-generating and air-conditioning equipment. After college, he worked as an engineer at his father's O'Brien Machinery Co. in Philadelphia and later became its president and chief executive. By the time he founded his own company, O'Brien had become active in government affairs. He was appointed by President Eisenhower to the Latin American Economic Trade Commission and later performed power grid studies for India, at the United Nations' request.

O'Brien retired to Fort Lauderdale in 1980 and devoted himself to a wide range of charity projects.