The Collaborative International Dictionary of English v.0.48:

Omicron \O*mi"cron\, n. [Written also omikron.] [NL., fr. Gr.
   [Omicron] [omicron]. See Micro-.]
   Lit., the little, or short, O, o; the fifteenth letter of the
   Greek alphabet.
   [Webster 1913 Suppl.]