In Greek (Ομικρον, Omikron) , Etruscan and Latin O stood for the vowel /o/. Although Semitic 'Ajin was used in some alphabets to transcribe [o], the sound value was usually consonantic: [?/] (as the Arabic letter 'Ajn).
A captital O is used in
complexity theory and
mathematics to describe the size of functions, using a notation called "
Big O notation".
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