After just one term in the state House, O'Brien successfully ran for Congress in 1973. O'Brien was a centrist who was popular among members of both parties. Following redistricting in 1982, he and fellow Republican Ed Derwinski were drawn into the same district. The redrawn district retained Derwinski's number, the 4th, and Derwinski had 14 years more seniority than O'Brien. However, the new district was geographically more O'Brien's district than Derwinski's. O'Brien won the primary, and then general elections. In April 1986, O'Brien discovered that he had prostate cancer, which forced him to give up his bid for an eighth term. He succumbed to the disease just two months later. Fellow Illinois Republican Congressman John E. Grotberg too died of cancer just four months later.
