Irish-born British radical, a leader of the Chartist working-class movement, sometimes known as the “Chartist schoolmaster.”

O'Brien was educated at Trinity College, Dublin, and moved to London in 1829, intending to practice at the English bar. In London he was quickly drawn into radical activities and later into working-class journalism, being editor of the radical Poor Man's Guardian (1831–35) and working on the Northern Star (1838–40). He was one of the most important and influential Chartists at that movement's convention in 1839. In 1850 he was joint founder of the National Reform League, which advocated socialist objectives. In his later years he wrote political poetry.