O

O is the fifteenth letter of the modern English alphabet and the fourth vowel of the Latin script, a rounded vowel letter whose uppercase and lowercase forms (O, o) have remained a plain circular outline from Phoenician times to the present. It is encoded in Unicode as U+004F LATIN CAPITAL LETTER O and U+006F LATIN SMALL LETTER O in the Basic Latin block, positions it shares with the original ASCII standard.^{1 • 2}

Origin and early history

The letter descends from Phoenician ʿayin, the sixteenth letter of the Phoenician alphabet, whose name meant "eye" and whose form was a simple circle. The Greeks adopted it as a vowel symbol, at first using it for both long and short o values. In many varieties of the Greek alphabet the same symbol also served for the long vowel that arose from contraction of two vowels or loss of a consonant, written ου in the Ionic alphabet.³

Later, a separate letter, omega (Ω), meaning "big O", was created for the long value, with the original circle, omicron (Ο), "little O", kept for the short value.⁵ The Greeks themselves did not use either name; the names omicron and omega are later assignments, though they appear even in modern Greek grammars.³

The related Semitic letters look very different but share the origin: the Aramaic and Hebrew ע arise from a circle left open at the top, a form traceable in Aramaic from the 5th or 6th century BC.³

The Latin alphabet was derived from the alphabet used by the Etruscans, who had adopted a Western variant of the classical Greek alphabet; it originally contained only 24 capital letters, and O passed into it unchanged in shape.¹¹ Across roughly three thousand years of borrowing, from Phoenician through Greek, Etruscan, and Latin, the letter has kept the same plain circular outline, which makes it among the most graphically stable letters in the Latin alphabet.⁴

Sound values

In standard English, the capital letter corresponds to the vowel sound /oʊ/, and the lowercase letter represents the rounded vowel heard in words such as "hot" or "go".^{1 • 2} English spelling gives O a wide range of values, including the short O, word-final long O, pre-consonantal long O, and O with the value of U, one reason English is described as a deep orthography (spelling system where letter-sound mapping is unpredictable) in which decoding relies on more than letter-by-letter reading.⁵

In Classical Greek, omicron was a short closed o and omega a long open-mid o; the two names are post-classical, the earlier name for the letter having simply been its sound.⁵

Extended and accented forms

Because the Latin script has been adapted to thousands of languages, O carries more diacritic combinations than almost any other base letter. The Latin-1 Supplement, which extends the basic 26 letter pairs of ASCII for the major languages of Europe, includes Ò, Ó, Ô, Õ, and Ö, each canonically decomposable as O plus a combining mark (grave, acute, circumflex, tilde, or diaeresis), and Ø, O WITH STROKE.⁶ Ø is an exception to decomposition: it does not decompose to O plus the combining long solidus overlay, so it must be handled as a distinct character.¹²

Latin Extended-A adds Ō (macron), Ŏ (breve), Ő (double acute), and Ǫ (ogonek, used in Sami, Iroquoian, and Old Icelandic), among others.⁷ Latin Extended Additional encodes the Vietnamese system, in which O combines with the hook above, dot below, and horn marks and with tone-marked circumflex forms such as Ố, Ồ, Ỗ, and Ộ.⁸

Language use varies in how these forms are treated. In the second-level Reference Label Generation Rules for the Latin script, ó appears in Spanish, Polish, Czech, Icelandic, and other languages, while ø is used in Danish and Faroese; the same rules block the Greek ο and Cyrillic о as cross-script homoglyphs of Latin o.¹⁰

Confusability with zero and with other scripts

Two confusions dominate the letter's modern typographic life. The first is with the digit zero: because O and 0 can look identical in many typefaces, type designers have developed conventions such as a narrower oval for zero or a slash or dot through its center. The distinction has no ancient precedent, since the Roman numeral system had no symbol for zero.⁴ Research on the problem includes a 1969 analysis by the psychologist Dirk Wendt in the Journal of Typographic Research, who found that a zero narrower than the letter O is more often interpreted correctly, and a proposal by Vartabedian (1969) to add a loop or stroke to the top of the O to distinguish it from zero.¹³

The second is cross-script homoglyphy. Latin O, Cyrillic О, and Greek Ο (omicron) are visually indistinguishable in most sans-serif fonts, a trio notorious in security research for homograph attacks on identifiers such as domain names.⁹ Registry-level rules respond by blocking the look-alike code points when a Latin label is registered.¹⁰

References

1. U+004F "O" Latin Capital Letter O, unicodecharacter.org
2. U+006F "o" Latin Small Letter O, unicodecharacter.org
3. "O", 1911 Encyclopædia Britannica, Wikisource
4. Alphabet Letters: O, Divinity Atlas
5. "O", Encyclopedia.com
6. The Unicode Standard, Version 17.0, C1 Controls and Latin-1 Supplement code chart
7. Latin Extended-A names list, Unicode 18.0.0
8. Latin Extended Additional names list, Unicode 18.0.0
9. Zero vs Letter O: Unicode Confusables and Homograph Attacks, SymbolFYI
10. Latin (script, full variant set), Second-Level Reference Label Generation Rules, IANA
11. Chapter 7, The Unicode Standard, Version 18.0.0
12. FAQ, Latin and Cyrillic Scripts, Unicode
13. CMBXSL10, TUGboat 34(2)

Topic: Encyclopedia › Society and history › Languages and linguistics › Writing and notation systems › Latin script

Initially written Sep 17, 2026 · Reviewed: Oct 25, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 25, 2026

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