Although Harrington was sympathetic to republicanism, he was a devoted friend of King Charles I and was briefly imprisoned shortly before the King was executed in 1649 in the course of the English Civil War. His views did not favourably impress Oliver Cromwell (Cromwell, Oliver), lord protector (1653–58) during the Commonwealth; Oceana was seized from its printer, and the intervention of Cromwell's daughter Elizabeth (Mrs. John Claypoole) was required to release the book for publication. Imprisoned in the early 1660s on a dubious charge of plotting against the restored monarchy under Charles II, Harrington was freed after his physical and mental health had been permanently impaired.

Oceana presents Harrington's vision of the ideal state—an aristocracy of limited, balanced powers. Harrington believed that democracy is most stable where a strong middle class exists and that revolution is a consequence of the separation of economic and political power. These beliefs particularly influenced U.S. Pres. Thomas Jefferson's (Jefferson, Thomas) democratic agrarianism and the antitrust policies of Theodore Roosevelt (Roosevelt, Theodore) and Woodrow Wilson (Wilson, Woodrow). Harrington also advocated the division of the country into landholdings of a specified maximum value, a referendum on each law proposed by the legislature, and a complicated scheme of rotation for public officials. His ideas are said to have been partly responsible for such U.S. political developments as written constitutions, bicameral legislatures, and the indirect election of the president.

An edition of Oceana prepared by Sten Bodvar Liljegren appeared in 1924.