OMICRON

'Omicron' (uppercase 'Ο', lowercase 'ο', literally "small o": ''o mikron'', micron meaning 'small' in contrast to ''omega'') 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 (number). This letter is equal to the Phoenician letter Ayin. It is also most similar to the modern English letter O, as in 'hot'.
In Unicode the capital letter is codepoint U+039F and the lowercase is U+03BF.
The upper-case letter of 'omicron' (O) was originally used as a symbol for

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