O'Brien was born in Chicago, Illinois to a middle-class Irish Catholic family. He attended a private Catholic school before entering Northwestern University. He is a member of the Sigma Chi Fraternity. O'Brien earned a law degree from Yale University. However, he did not get to practice law until returning from World War II. During the war, O'Brien rose to the rank of lieutenant while fighting in Germany. Beginning in 1946, he practiced law privately. In 1970, he was appointed to the state's Legislative Advisory Committee, and in 1971 he was elected to the Illinois House of Representatives.
After just one term in the state House, O'Brien successfully ran for Congress in 1973. His popularity surged because of his personal endorsement from retiring representative Leslie Arends. O'Brien, like his predecessor, was a near centrist, who was popular among members of both parties. In April 1986, O'Brien discovered that he had prostate cancer, which forced him to resign from that year's election. He succumbed to the disease just two months later.