The Greek origin rhetorical device called "polyptoton" is a means of dramatically and persuasively enhancing meaning in speech or writing by using a word and a cognate of the word in the same sentence or in close proximity. A cognate of a word is a word that shares a common etymological origin with the first. Etymology is origin of a word, the history of a word or a morpheme. An example of cognation is eat and its cognate eating or provide and its cognate provider. In the quote above polyptoton occurs between the word oppressor and its cognate oppressed. These words share the same etymology and are in close proximity to each other. In addition, the polyptoton enhances the rhetorical effect by increasing the dramaticality and the persuasiveness of the statement. Polyptoton also draws an irony or a paradox to the surface. Nation Master Encyclopedia has some good examples of polyptotons that draw ironies and paradoxes in situations.
John F. Kennedy: "Not as a call to battle, though embattled we are."
Franklin Delano Roosevelt: "Let me assert my firm belief that the only thing we have to fear is fear itself."
Sir Philip Sidney: "Thou art of blood, joy not to make things bleed."
The first draws out the irony of the situation Kennedy is addressing. FDR's polyptoton reveals a paradox. Sir Philip Sydney's shows in reverse the irony of someone of blood wishing to make one bleed.