O is a letter of the Cyrillic script.

O commonly represents the close-mid back rounded vowel /o/, like the pronunciation of ⟨o⟩ in "go".

History

The Cyrillic letter O was derived from the Greek letter Omicron

Omicron

Omicron is the 15th letter of the Greek alphabet. In the system of Greek numerals it has a value of 70. It is rarely used in mathematics because it is indistinguishable from the Latin letter O and easily confused with the digit 0...

 (Ο ο).

The name of O in the Early Cyrillic alphabet

Early Cyrillic alphabet

The Early Cyrillic alphabet is a writing system developed in the First Bulgarian Empire in the 9th or 10th century to write the Old Church Slavonic liturgical language...

 was (onŭ), meaning "he" or "it".

In the Cyrillic numeral system

Cyrillic numerals

The Cyrillic numerals are a numbering system derived from the Cyrillic script, used by South and East Slavic peoples. The system was used in Russia as late as the early 18th century when Peter the Great replaced it with Arabic numerals....

, O had a value of 70.

Form

The Cyrillic letter O looks exactly like

Homoglyph

In typography, a homoglyph is one of two or more characters, or glyphs, with shapes that either appear identical or cannot be differentiated by quick visual inspection. This designation is also applied to sequences of characters sharing these properties....

 the Latin letter O

O

O is the fifteenth letter and a vowel in the basic modern Latin alphabet.The letter was derived from the Semitic `Ayin , which represented a consonant, probably , the sound represented by the Arabic letter ع called `Ayn. This Semitic letter in its original form seems to have been inspired by a...

 ⟨O o⟩ and the Greek letter Omicron

Omicron

Omicron is the 15th letter of the Greek alphabet. In the system of Greek numerals it has a value of 70. It is rarely used in mathematics because it is indistinguishable from the Latin letter O and easily confused with the digit 0...

 ⟨Ο ο⟩.

Usage

In Russian

Russian language

Russian is a Slavic language used primarily in Russia, Belarus, Uzbekistan, Kazakhstan, Tajikistan and Kyrgyzstan. It is an unofficial but widely spoken language in Ukraine, Moldova, Latvia, Turkmenistan and Estonia and, to a lesser extent, the other countries that were once constituent republics...

, O is used word-initially, after another vowel, and after non-palatalized consonants. Because of a vowel reduction processes

Vowel reduction in Russian

Vowel reduction in Russian differs in the standard language and in dialects. Several ways of reduction are distinguished.There are five vowel phonemes in Standard Russian. Vowels tend to merge together when they are unstressed. The vowels and have the same unstressed allophones for a number of...

, the Russian /o/ phoneme may have a number of pronunciations in unstressed syllables, including [ɐ] and [ə].

Related letters and other similar characters

• Ο ο : Greek letter Omicron
• O o : Latin letter O
  O
  O is the fifteenth letter and a vowel in the basic modern Latin alphabet.The letter was derived from the Semitic `Ayin , which represented a consonant, probably , the sound represented by the Arabic letter ع called `Ayn. This Semitic letter in its original form seems to have been inspired by a...
  
  
• Ё ё : Cyrillic letter Yo : Cyrillic letter Broad On : Cyrillic letter O with diaeresis : Cyrillic letter Oe : Cyrillic letter Oe with diaeresis : Cyrillic letter O-hook

Computing codes

character	О		о
Unicode name	CYRILLIC CAPITAL LETTER O		CYRILLIC SMALL LETTER O
character encoding	decimal	hex	decimal	hex
Unicode Unicode Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...	1054	041E	1086	043E
UTF-8 UTF-8 UTF-8 is a multibyte character encoding for Unicode. Like UTF-16 and UTF-32, UTF-8 can represent every character in the Unicode character set. Unlike them, it is backward-compatible with ASCII and avoids the complications of endianness and byte order marks...	208 158	D0 9E	208 190	D0 BE
Numeric character reference Numeric character reference A numeric character reference is a common markup construct used in SGML and other SGML-related markup languages such as HTML and XML. It consists of a short sequence of characters that, in turn, represent a single character from the Universal Character Set of Unicode...	О	О	о	о
KOI8-R KOI8-R KOI8-R is an 8-bit character encoding, designed to cover Russian, which uses the Cyrillic alphabet. It also happens to cover Bulgarian, but is not used since CP1251 is accepted. A derivative encoding is KOI8-U, which adds Ukrainian characters...  and KOI8-U KOI8-U KOI8-U is an 8-bit character encoding, designed to cover Ukrainian, which uses the Cyrillic alphabet. It is based on KOI8-R, which covers Russian and Bulgarian, but replaces eight graphic characters with four Ukrainian letters Ґ, Є, І, and Ї in both upper case and lower case.In Microsoft Windows,...	239	EF	207	CF
CP 855 Code page 855 Code page 855 is a code page used under MS-DOS to write Cyrillic script. This code page is not used much.-Code page layout:...	215	D7	214	D6
Windows-1251 Windows-1251 Windows-1251 is a popular 8-bit character encoding, designed to cover languages that use the Cyrillic alphabet such as Russian, Bulgarian, Serbian Cyrillic and other languages...	206	CE	238	EE
ISO-8859-5	190	BE	222	DE
Mac Cyrillic MacCyrillic encoding The Macintosh Cyrillic encoding is used in Apple Macintosh computers to represent texts in the Cyrillic script.Each character is shown with its equivalent Unicode code point and its decimal code point. Only the second half of the table is shown, the first half being the same as ASCII....	142	8E	238	EE