SHAPE: Consistent shape in all ancient cities of the Western-Eastern greek alphabet groups.

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SOUND: Till 403/2 a.e.v. the eastern alphabets used collectively Omicron for the sounds:
1) /o/ short (as it is today),
2) /o/ long [o:], which later took a form of its own letter, the omega and
3) an /o/ diphthong which sounded like /u/ and later was represented by OY. After hellenistic times the omega and the omicron, are pronounced similarly.

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NAME: ETYMOLOGY: From northsemitic (phoenician): 'ayin = (the) eye. The letter's hellenistic name was «o», affected by the OY lettering of sound [u]. Later, to make the antithesis to Omega Ω, it was called...
NAME: SPELLING: «   μικρóv» [`o] [mi`kron] = O small), hence the letter-name
«μικρov» [`omikron], in contrast to O Mega (=O big) → Omega.

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COMBINATIONS: Check OI [i] erasmic /oi/ and OY [u].

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MORE GREEK WORDS STARTING WITH O
•όνoμα=name - (syn)onym •oξύ=sharp,intense - oxy(gen) •oλίγoν=a few - olig(archy) •oρθόν=correct,straight - ortho(graphy) •όργανoν=intrument - organ •oκτώ=eight - octave •oδύσσεια-odyssey
omikron with daseia: •oρίζων = horizon