Dennis or Denis O'Brien may refer to:
Dennis O'Brien (ice hockey) (born 1949), professional ice hockey player in the National Hockey League Dennis M. O'Brien (born 1952), former speaker, PA House of Representatives, and member, 167th District Denis O'Brien (born 1958), Irish entrepreneur, charman of Digicel Denis O'Brien (footballer) (born 1952), Australian footballer for Collingwood Denis O'Brien (police officer) (1899–1942), veteran of the Easter Rising and the Irish Republican Army Denis O'Brien (politician) (1837–1909), New York state attorney general Denis O'Brien (producer) (1941–2021), co-founder of HandMade Films with George Harrison Denis Patrick O'Brien (born 1939), English economist Denny O'Brien (born 1973), American columnist, journalist and radio commentator G. Dennis O'Brien (born 1931), former president of the University of Rochester
See also Dennis O'Bryen (1755–1832), Irish dramatist and political pamphleteer