ASCII Character ΟGreek capital letter Omicron
Omicron (/ˈoʊmɪkrɒn, ˈɒmɪkrɒn, oʊˈmaɪkrɒn/; uppercase Ο, lowercase ο, Greek: όμικρον) is the 15th letter of the Greek alphabet. This letter is derived from the Phoenician letter ayin: Phoenician ayin.svg. In classical Greek, omicron represented the close-mid back rounded vowel IPA: [o] in contrast to omega which represented the open-mid back rounded vowel IPA: [ɔː] and the digraph ου which represented the long close-mid back rounded vowel IPA: [oː]. In modern Greek, both omicron and omega represent the mid back rounded vowel IPA: [o̞] or IPA: [ɔ̝]. Letters that arose from omicron include Roman O and Cyrillic O. The word literally means "little O" (o mikron) as opposed to "great O" (ō mega). In the system of Greek numerals, omicron has a value of 70. In addition to its use as an alphabetic letter, omicron is occasionally used in technical notation, but its use is limited since both upper case and lower case (Ο ο) are indistinguishable from the Latin letter "o" (O o) and difficult to distinguish from the Arabic numeral "zero" (0).