Ο

Omicron in a capital letter Etymologicum Magnum (1499).

Omicron (uppercase Ο, lowercase ο; Ancient Greek call οὖ oû /ûː/, in modern Greek όμικρον ómikron /ˈomikron/, lit. 'o small') is the fifteenth letter of the alphabet Greek.

The word omicron is a medieval invention to distinguish it from omega, pronounced in ancient Greek as a long, mid-opening o. Before that, the omicron was simply called οὖ (oû).

Omicron gave rise to the letters o Latin and o Cyrillic, which have similar phonetic value and the same appearance in both upper and lower case.

In the Greek numeral system it has a value of 70 (Οʹ).

History

Despite its apparent simplicity, the letter omicron is clear evidence of two historical changes to the Greek alphabet.

On the one hand, omicron makes evident the adaptations that the Greeks had to make in the Phoenician alphabet to adapt its letters to Greek phonology. Both the shape and the alphabetical position refer to the Phoenician letter ʿayn (𐤏), a letter from which the Hebrew letters ʿayn ע and Arabic ع also come. However, both the name and the pronunciation of omicron are totally different. This is explained because the pharyngeal fricative consonant /ʕ/ that represented ʿayn was redundant for writing Greek, while it lacked signs to represent the vowels, so they reused the letter to represent the vowel O. Due to this radical change, the name ʿayn it is not adapted to Greek (like the name of almost all other letters) and is named simply with the sound it represented: "ο".

The other historical clue is in the modern name of the letter, which went from being «o» to being «ómicron» (in Greek ό μικρον, o micron, that is to say «or small») to contrast with "omega" (ὦ μέγα, ō mega, "or big"). The change is due to the reduction of the vowels in Modern Greek compared to Classical Greek. There were formerly three vowel sounds similar to O: rounded back semiclosed ([o]), represented as ⟨Ο⟩; long rounded back semi-closed ([oː]), represented as ⟨ΟΥ⟩, and long rounded back semi-open ([ɔː]), represented as ⟨Ω⟩. Since Byzantine times omicron and omega have been pronounced the same and ου is pronounced as u.

Epigraphic variants

Omicron was used in all archaic Greek alphabets generally always resembling its Phoenician, circular model. Exceptions included the alphabets of Paros and Thasos, where omicron was more reminiscent of the later letter omega, and the alphabets of Milos and other southern Cycladic islands, where omicron resembled the later letter C of the Latin alphabet.

In archaic epigraphic sources the following variants appear:

Use

While the other Greek letters are used as symbols in numerous scientific fields, omicron is rarely used because it has the distinction of being the only Greek letter identical in both its uppercase and lowercase form to its Latin counterpart, the letter O, apart from to be confused with the number zero. Still, it has a few uses:

• In theory of computer complexity, the asymptotic growth rate of a function according to notation of or capital.
• In astronomy it was used to refer to the fifteenth stars of its constellations, according to the standard star nomenclature, for example Omicron Andromedae, Omicron Ceti and Omicron Persei.
• In medicine, the Omicron variant of SARS-CoV-2. The World Health Organization (WHO) uses the Greek alphabet to describe the disturbing variants of SARS-CoV‐2, the virus that causes COVID-19 On 26 November 2021, Omicron was assigned to the worrying variant B.1.529

Unicode

• Greek and Coptic

• Maths