Born in St. Louis, Missouri, O'Neill attended the common schools.
He studied law.
He was admitted to the bar in 1870 and commenced practice in St. Louis.
He engaged in the manufacture of gold pens.
He served as a member of the Missouri House of Representatives from 1872 to 1878.
He served as a member of the municipal assembly from 1879 to 1881.
O'Neill was elected as a Democrat to the Forty-eighth, Forty-ninth, and Fiftieth Congresses (March 4, 1883 – March 3, 1889).
He served as chairman of the Committee on Expenditures on Public Buildings (Forty-ninth Congress), Committee on Labor (Forty-ninth and Fiftieth Congresses).
He was an unsuccessful candidate for reelection in 1888 to the Fifty-first Congress.
O'Neill was elected to the Fifty-second Congress (March 4, 1891 – March 3, 1893).
He successfully contested the election of Charles F. Joy to the Fifty-third Congress (April 3, 1894 – March 3, 1895).
He was not a candidate for renomination in 1894.
He resumed the practice of law.
He died in St. Louis, Missouri, February 19, 1898.
He was interred in Calvary Cemetery.
* Alternately named Economic and Educational Opportunities in 104th Congress and Education and the Workforce in 105th through 109th and 112th through 115th Congresses.
