He served in the United States Navy from 1919–1923, after which he became engaged in the real estate business in Newark. He was an unsuccessful candidate for election in 1934 to the Seventy-fourth Congress, and was elected as a Democrat to the Seventy-fifth Congress, serving in office for a single term from January 3, 1937 – January 3, 1939. O'Neill was an unsuccessful candidate for reelection in 1938 to the Seventy-sixth Congress.
After his departure from Congress, he served as a lieutenant in the United States Naval Reserve in 1939 and 1940, and served as a captain in the United States Army Quartermaster Corps in 1942 and 1943. He was commissioner of the Essex County Board of Taxation 1940-1945. He was a realtor and mortgage broker in Newark until his death on December 12, 1948.
He was interred in Holy Sepulchre Cemetery in East Orange, New Jersey.