ランダウの記号
The upper-case letter of omicron (O) was originally used in mathematics as a symbol for Big O notation (representing a function's asymptotic growth rate), but has fallen out of favor because omicron is indistinguishable from the Latin letter O and easily confused with the digit zero (0). Where "Big O" notation is still used, Omicron is generally replaced with a script- or caligraphic-form Latin letter "Oh" (formula_1).