William James O'Brien (May 28, 1836 – November 13, 1905) was a U.S. Congressman from the third district of Maryland, serving two terms from 1873 until 1877. Born in Baltimore, Maryland, O'Brien attended the common schools and pursued classical studies in the old St. Mary's College of Baltimore. He studied law, and was admitted to the bar in 1858, commencing practice in Baltimore. He was elected as a Democrat to the Forty-third and Forty-fourth Congresses, serving from March 4, 1873, until March 3, 1877, but was not a candidate for renomination in 1876.