The Greek letter written as "o" or "O" for denoting little-o notation and big-O notation respectively. Little-o notation denotes a theoretical measure of the execution of an algorithm, usually the time or memory needed, given the problem size n, which is usually the number of items. The big-O notation denotes a theoretical measure of the execution of an algorithm, usually the time or memory needed, given the problem size n, which is usually the number of items.