Omicron

Omicron (uppercase/lowercase Ο ο) is a letter of the Greek alphabet. It is used to represent the "o" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 70. Letters that came from it include the Roman O and Cyrillic О.
In mathematics and computer science, the uppercase Ο is used in big-O notation, while the lowercase ο is used in little-o notation (a variant of big-O notation).