Omicron

"There is no reason to think that the sounds represented by [ε and ο] were ever other than short mid vowels, front* and back* respectively, i.e. rather like the vowels of English pet and German Gott.[1]  The view that they were of a specially close mid quality, i.e. [ẹ], [ọ], as in French gai, beau, is probably mistaken."

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[1] The vowel of English pot is decidedly less accurate, being fully open rather than mid.

     W. Sidney Allen, Vox Graeca:  a guide to the pronunciation of classical Greek, 3rd ed. (New York:  Cambridge University Press, 1987), 63, underscoring original.