Daniel O'Leary may refer to:
Daniel O'Leary (chemist), American chemist Daniel O'Leary (mobster) "Danny" (died 1928), Philadelphia mobster Daniel O'Leary (Irish nationalist politician) (1875–1954), Irish Parliamentary Party MP for West Cork (1916–18) Daniel Florence O'Leary (c.1802–1854), Irish-born South American brigadier general Daniel O'Leary (Fine Gael politician) (1877–1951), Irish Cumann na nGaedhael / Fine Gael politician from Cork Dan O'Leary (American football) (born 1977), American football player Dan O'Leary, baseball player K. Daniel O'Leary (born 1940), American psychologist