|"We must've been through hundreds of books already and we can't find him anywhere —"|
Known family members
- Cormac O'Brien — one of the original twelve American Aurors
- Harvey O'Brien — a Hufflepuff student at Hogwarts School of Witchcraft and Wizardry in the 1990s
O'Brien is an Anglicised form of the Irish Gaelic surname Ó Briain, meaning "descendant of Brian." Brian is an Irish male given name thought to derive from the Celtic element bre, meaning "hill," or, by extension, "high" or "noble."