O'Casey is a common variation of the Gaelic cathasaigh, meaning vigilant or watchful, with the added anglicized prefix O' of the Gaelic Ó, meaning grandson or descendant. At least six different septs used this name, primarily in the Counties of Cork and Dublin.

People with the surname O'Casey include:

• Seán O'Casey (1880–1964), Irish playwright
• Eileen O'Casey (1900–1995), Irish actress, author, and wife of Sean O'Casey
• Breon O'Casey (1928–2011), son of Seán and Eileen O'Casey
• Lance O'Casey, cartoon character
• Ronan O'Casey (1922–2012), Canadian actor and producer
• O'Casey (O'Cathasaigh), chief of Saithne, now Sonagh, in Westmeath; see Tuite Baronets

• Casey (surname)
• Casey (disambiguation)