О (О о; italics: О о or О о; italics: О о) is a letter of the Cyrillic script.

The letter most commonly represents the sound /ɔ/, like the pronunciation of ⟨au⟩ in "aura". In Russian and Serbo-Croatian, it represents the sound /o/.

The Cyrillic letter О was derived from the Greek letter Omicron (Ο ο).

In modern-style typefaces, the Cyrillic letter O looks exactly like the Latin letter O ⟨O o⟩ and the Greek letter Omicron ⟨Ο ο⟩.

Historical typefaces (like poluustav (semi-uncial), a standard font style for the Church Slavonic typography) and old manuscripts represent several additional glyph variants of Cyrillic O, both for decorative and orthographic (sometimes also "hieroglyphic") purposes, namely:

In Russian, O is used word-initially, after another vowel, and after non-palatalized consonants. Because of a vowel reduction processes, the Russian /o/ phoneme may have a number of pronunciations in unstressed syllables, including [ɐ] and [ə]. It is the most common letter in the Russian language.

In Macedonian, Bulgarian, Ukrainian and Belarusian, the letter represents the sound /ɔ/.

In Tuvan the Cyrillic letter can be written as a double vowel.