The term predates the holocaust. Wikipaedia has this to say about it (since I don't have access to the OED):
The term was originally used for the Venetian Ghetto in Venice, Italy, as early as 1516, to describe the part of the city where Jews were restricted to live and thus segregated from other peoples. However, early societies may have formed their own versions of the same structure; words resembling "ghetto" in meaning appear in Hebrew, Yiddish, Italian, Germanic, Old French, and Latin.