The word is fairly recent: the Oxford English Dictionary only included it December 2004. But it has been in use since at least 1995. At least some writers compare it to the more familiar underclass:
We now have a quite new phenomenon in the history of the republic: two radically isolated sectors of the population, the underclass and the overclass. Both are in an adversarial posture toward the great majority of Americans, the overclass by virtue of ambition and unbounded self-esteem, the underclass by virtue of social incompetence and anomie. Between the two there is a fearful symmetry on many scores, but their service to each other is far from equal.
Adler, Jerry (July 31, 1995). "The Rise of the Overclass; The Overclass 100". Newsweek126 (5): 32–46. – Newsweekcover story on "How the new elite scrambled up the merit ladder—and wants to stay there any way it can."