Walter Howard "Hod" O'Brien (January 19, 1936 – November 20, 2016) was an American jazz pianist.
O'Brien was born in Chicago. He attended the Hotchkiss School and then studied at the Oberlin Conservatory and at the Manhattan School of Music (1954–1957). He began playing professionally in 1950, and substituted for Randy Weston in 1955. He led his own group in Lenox, Massachusetts during 1956–57, then joined Oscar Pettiford in 1957–58, and J.R. Monterose/Elvin Jones in 1958–59. Between 1960 and 1963 he played with Phil Woods, Freddie Hubbard, Charlie Rouse, and Lee Konitz. In 1964 he enrolled at Columbia University, where he studied mathematics and psychology, achieving his bachelor's degree in 1969; he then took a position in computer programming at New York University during 1969–1974, and studied computer music under Hall Overton and Charles Wuorinen.
O'Brien ran the St. James Infirmary jazz club in New York City in 1974–75 and played in the house band alongside Beaver Harris and Cameron Brown; this group accompanied Roswell Rudd, Sheila Jordan, Chet Baker, Zoot Sims, Al Cohn, Pepper Adams, and Archie Shepp, among others. He played with Marshall Brown (1975), Russell Procope and Sonny Greer (1977), Joe Puma (1977–1982, at Gregory's in New York City), and Stephanie Nakasian (1982). He later married Nakasian. (Their daughter is singer Veronica Swift.) He taught at Turtle Bay Music School 1972–1975, and in the 1990s at the University of Virginia.
Hod O'Brien continued playing actively while he underwent treatment for cancer. He died at the age of 80 on November 20, 2016.