Omicron instance of Greek letter
Omicron (; uppercase Ο, lowercase ο, Greek: όμικρον) is the 15th letter of the Greek alphabet. This letter is derived from the Phoenician letter ayin: . 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.Greek numerals
There were several systems for writing numbers in Greek; the most common form used in late classical era used omicron (either upper or lower case) to represent the value 70.
More generally, the letter omicron is used to mark the fifteenth ordinal position in any Greek-alphabet marked list. So, for example, in Euclid's Elements, when various points in a geometric diagram are marked with letters, it is effectively the same as marking them with numbers, each letter representing the number of its place in the standard alphabet.Astronomy
Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position. Such stars include Omicron Andromedae, Omicron Ceti, and Omicron Persei.
In Claudius Ptolemy's (c. 100–170) Almagest, tables of sexagesimal numbers  1 ... 59  are represented in the conventional manner for Greek numbers: ′α  ′β ... ′νη ′νθ  . Since the letter omicron [which represents 70 (′ο) in the standard system] is not used in sexagesimal, it is re-purposed to represent an empty number cell. In some copies, zero cells were just left blank (nothing there, value is zero), but to avoid copying errors, positively marking a zero cell with omicron was preferred, for the same reason that blank cells in modern tables are sometimes filled-in with a long dash (—). Both an omicron and a dash imply that "this isn't a mistake, the cell is actually supposed to be empty". By coincidence, the ancient zero-value omicron (′ο) resembles a modern Hindu-Arabic zero (0).
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