Michael Joseph "Joe" O'Sullivan (November 4, 1906 – October 13, 1971) was an American jazz pianist.
Sullivan was the ninth child of Irish immigrant parents. He studied classical piano for 12 years and at age 17, he began to play popular music in a club where he was exposed to jazz. He graduated from the Chicago Conservatory and was an important contributor to the Chicago jazz scene of the 1920s. Sullivan's recording career began late in 1927 when he joined McKenzie and Condon's Chicagoans. Other musicians included Jimmy McPartland, Frank Teschemacher, Bud Freeman, Jim Lanigan and Gene Krupa. In 1933, he joined Bing Crosby as his accompanist, recording and making many radio broadcasts. After suffering for two years with tuberculosis, he briefly rejoined Bing Crosby in 1938 and the Bob Crosby Orchestra in 1939.
By the 1950s, Sullivan was largely forgotten, playing solo in San Francisco. Marital difficulties and excessive drinking caused Sullivan to become increasingly unreliable and unable to keep a steady job, either as band member or soloist.