Omicron: Greek letter Ο ο — form, history, and uses

Form and phonetics

Visually Omicron resembles the Latin letter O and the Cyrillic О; these resemblances reflect shared ancestry and later borrowings rather than identical characters. In classical phonology Omicron marked a short /o/ sound, contrasted with Omega for the long /ɔː/ or /oː/ vowel. In Modern Greek both letters represent an /o/ sound. For discussion of the language stages in which it appears, consult sources on Ancient Greek.

Origins and development

The Greek vowel shapes descend from earlier Semitic letterforms; the glyph that became Omicron traces back to the Phoenician prototype for an "ayin"-type sign. As the Greek alphabet was adopted and adapted, Omicron gave rise to several letters in other scripts: the Roman O used in Latin and the Cyrillic О both derive from this Greek form. In the Greek alphanumeric scheme (Greek numerals), Omicron has the value 70 — see Greek numerals.

Uses and conventions

• Typography and encoding — Although Omicron looks identical to the Latin O in many fonts, Unicode assigns separate code points to Greek and Latin letters so that linguists and typographers can distinguish them in text.
• Astronomy — Greek letters, including Omicron, are used in the Bayer designation of stars (for example, stars named Omicron in various constellations).
• Mathematics and computer science — Symbols that look like Omicron are used in asymptotic notation: the capital form appears in big-O usage and the small form in little-o usage. For general mathematical context see mathematics and for computational contexts see computer science. The conventional shorthand for growth-rate notation is often referenced as big-O notation.

Because the glyph is visually simple and common, it is often a source of typographic confusion with the numeral zero and with letters from related scripts; careful encoding and font choice avoid accidental mixes of Greek, Latin and Cyrillic forms. Omicron is a small but persistent element of written and scientific culture, bridging alphabetic history, numeric systems, and technical terminology.