William Smith O'Brien (January 8, 1862 – August 10, 1948) was a lawyer and Democratic politician from West Virginia who served as a United States representative from 1927 to 1929.

O'Brien was born in Audra, near Philippi in Barbour County, Virginia (now West Virginia). He attended the common schools, the Weston Academy, and the West Virginia University at Morgantown. He worked as a laborer on farms, in brick yards, and on public works. He also taught school and worked as an editor. He graduated from the law school of the University of West Virginia at Morgantown in 1891. He was admitted to the bar the same year and commenced the practice of law in Buckhannon, West Virginia in Upshur County in 1892.

O'Brien served as a captain in the West Virginia National Guard in 1894 and 1895. He served as judge on the twelfth judicial circuit court of West Virginia from 1913 to 1919. In 1926, he was elected as a Democrat to the 70th United States Congress (March 4, 1927 – March 3, 1929). He was an unsuccessful candidate for re-election in 1928 to the Seventy-first Congress and resumed the practice of law. He was elected to four successive terms as Secretary of State of West Virginia starting in 1932. He served as West Virginia Secretary of State until his death in Buckhannon, West Virginia in 1948 and was buried at Heavner Cemetery.

• West Virginia & Regional History Center at West Virginia University, William Smith O'Brien, Papers

 This article incorporates public domain material from the Biographical Directory of the United States Congress

• United States Congress. "William S. O'Brien (id: O000018)". Biographical Directory of the United States Congress.

v t e Members of the U.S. House of Representatives from West Virginia
1st district	Blair C. Hubbard Duval J. J. Davis B. Wilson Goff Pendleton Atkinson Pendleton Dovener W. Hubbard J. W. Davis Neely Rosenbloom Bachmann Ramsay Schiffler Ramsay Schiffler Neely Love Ramsay R. Mollohan Moore R. Mollohan A. Mollohan McKinley C. Miller
2nd district	Brown Sr. Latham Kitchen McGrew Hagans Faulkner Martin Hoge W. Wilson Dayton T. Davis Sturgiss Brown Jr. Bowers Allen Bowman Randolph M. Snyder Staggers Sr. Benedict Staggers Jr. Wise Capito Mooney
3rd district	Whaley Polsley Witcher Hereford Kenna C. Snyder Alderson Huling Dorr Johnston Gaines Littlepage Avis Littlepage Reed Wolverton O'Brien Wolverton Hornor Edmiston Rohrbough Bailey Rohrbough Bailey Slack Hutchinson Staton Wise Rahall Jenkins C. Miller
4th district	Gibson C. Hogg Jackson C. Smith Capehart W. Miller Freer Hughes Woodyard Hamilton Moss Woodyard Johnson Woodyard Hughes R. Hogg Johnson Ellis Burnside Neal Burnside Neal Hechler Rahall
5th district	Hughes Cooper Goodykoontz Lilly Strother Shott John Kee E. Kee James Kee
6th district	Littlepage Echols Taylor England J. Smith Hedrick Byrd Slack
At-large	Sutherland