O

The letter O is used for a number of different purposes in mathematics. The double-struck  is sometimes used to represent octonions. The symbols  (sometimes called the O-symbol, O-notation, or "big-O") and  (sometimes called "little-o") are a type of asymptotic notation collectively known as Landau symbols.

REFERENCES:

de Bruijn, N. G. Asymptotic Methods in Analysis. New York: Dover, pp. 3-10, 1981.