Omicron (upper case Ο, lower case ο, literally "small o": o mikron, micron meaning 'small') is the 15th letter of the Greek alphabet. In the system of Greek numerals it has a value of 70. It is not used in mathematics because it is indistinguishable from the Latin letter O. Indeed, it is not widely used because of confusion with the digit 0.

The upper-case letter Ο is used as a symbol for:

• The Big O notation, representing the asymptotic rate of growth of a function in mathematics.

Greek letters | Vowel letters

Ο | Omikron (lizherenn) | Омикрон | Òmicron | Omikron | Omikron | Όμικρον | Ο | Omicron | Omicron | Omicrón | Ο | Omicron (lettera) | אומיקרון | Omikron | Ο | Omikron | Omikron | Ο | Omikron | Ο