Olive Clio Hazlett (October 27, 1890 – March 8, 1974) was an American mathematician who spent most of her career working for the University of Illinois.

After receiving her doctoral degree Hazlett was awarded an Alice Freeman Palmer Fellowship by Harvard, which allowed her to research invariants of nilpotent algebras at Wellesley College for the next year.

In 1935 she wrote to the chair of the mathematics department complaining that the service courses left her no time for research, but her teaching schedule was not changed and by December 1936 she took a sick leave after having a mental breakdown from the stress of her job.

In 1940, she was appointed a member of the American Mathematical Society's Cryptanalysis Committee, for which she worked until the end of World War II.

She maintained her teaching job for most of this period, though, (except for taking leave in 1944-45) and went to great lengths to keep her Cryptanalysis Committee work secret.