Letter between xi and pi (7)

I believe the answer is:

omicron

(Other definitions for omicron that I've seen before include "letter from 13 [HELLENE]" , "character in Europe" , "character in Sophocles" , "old letter" , "Moronic (anag) - Greek letter" .)

I've seen this clue in The Wall Street Journal and the Sydney Morning Herald.

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