Omicron Symbol (ο): Meaning, Uses and How to Type It

Omicron is the fifteenth letter of the Greek alphabet and the one that mathematicians almost never use: its lowercase ο is indistinguishable from a Latin o and very close to the digit 0. The name became famous worldwide in 2021, when the World Health Organization gave it to a SARS-CoV-2 variant, and computer scientists still meet it in big-O notation, which Donald Knuth proposed reading as "big omicron."

This guide explains what omicron means and how to type ο and Ο on any device, in HTML and in LaTeX, where it has no command of its own.

Omicron Definition

Omicron (noun): the fifteenth letter of the Greek alphabet, written Ο (capital) and ο (small), pronounced "OM-ih-kron" or "OH-mih-kron" in English and "OH-mee-kron" in modern Greek. It comes after xi (ξ) and before pi (π), stood for 70 in the Greek numeral system, and is the ancestor of the Latin O.

The name is literally "o mikron," small o, coined by Byzantine grammarians to separate it from omega, "o mega," big o, once the two vowels had come to sound the same. For all 24 letters with their LaTeX commands, see the Greek alphabet in LaTeX lesson, and for how each letter is used as a math symbol, see Greek letters as math symbols.

Omicron Symbol (ο): Copy and Paste

Copy both letters from the box below (use the copy button), or take the code you need from the table:

1ο Ο
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The Greek ο (U+03BF) and the Latin o (U+006F) look the same in nearly every font but are different characters, so a search for one will not find the other. Use the Greek letter in Greek text and the Latin letter in formulas.

What the Omicron Symbol Means

The Omicron Variant of SARS-CoV-2

On 26 November 2021 the World Health Organization named the SARS-CoV-2 lineage B.1.1.529 "Omicron" and classified it as a variant of concern. The WHO was naming variants with Greek letters in order; it skipped nu, which could be confused with "new," and xi, a common surname. Its descendants, such as BA.2 and XBB, dominated the years that followed.

Big-O Notation in Computer Science

Big-O notation, as in O(n log n), was introduced by Paul Bachmann and Edmund Landau with a Latin O for "Ordnung" (order). In his 1976 paper "Big Omicron and Big Omega and Big Theta," Donald Knuth suggested reading the O as a capital omicron so that it would match the Greek Ω and Θ he defined alongside it. Either way, it is typed as a Latin O.

Star Names in Astronomy

Bayer designations label the stars of a constellation with Greek letters, and omicron is one of them. Omicron Ceti, better known as Mira, is a red giant whose brightness varies over about 332 days, the first periodic variable star recognised.

Why Omicron Is Rare in Mathematics

Every other lowercase Greek letter has a LaTeX command, but omicron does not: ο is visually identical to o, and in handwriting it is easy to confuse with 0 and with the degree sign. Mathematicians therefore skip it when choosing variable names and use ω, θ or φ instead.

How to Type Omicron

Windows: omicron has no short Alt code in the old IBM character set. In Word, type 03BF and press Alt + X; elsewhere, open Character Map and search "omicron."

Mac: there is no Option-key shortcut. Add the Greek input source in System Settings > Keyboard > Input Sources and press O for ο or Shift + O for Ο, or open the Character Viewer with Control + Command + Space and search "omicron."

iPhone and Android: add the Greek keyboard (Settings > General > Keyboard > Keyboards on iOS, or Gboard's language settings), switch with the globe key and tap O.

Microsoft Word: type 03BF and press Alt + X (039F for Ο), or use Insert > Symbol > More Symbols with the Greek and Coptic subset.

Google Docs: go to Insert > Special characters and search "omicron."

HTML: write ο for ο and Ο for Ο, or the numeric forms ο and Ο.

Omicron in LaTeX

Standard LaTeX and amsmath define no \omicron or \Omicron, and typing them gives an "Undefined control sequence" error. In math mode write o for lowercase omicron and O (or \mathrm{O} for an upright capital). If you want the command for readability, define it yourself, as in the example below.

A complete example you can compile:

1\documentclass{article}
2\usepackage{amsmath}
3\newcommand{\omicron}{o}
4\begin{document}
5Merge sort runs in $O(n \log n)$ time, and $f(n) = o(n^2)$.
6\[
7  T(n) = 2\,T(n/2) + O(n) \quad\Rightarrow\quad T(n) = O(n \log n)
8\]
9With the macro, $\omicron$ prints a lowercase omicron.
10\end{document}
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Related symbols and guides

• Greek letters reference — every symbol in the category with its LaTeX command
• Greek alphabet in LaTeX — how to typeset these symbols in a document
• Xi symbol (ξ)
• Pi symbol (π)

Omicron Symbol FAQ

What is the omicron symbol?

The omicron symbol is the Greek letter omicron: ο in lowercase (U+03BF) and Ο in uppercase (U+039F). It is the fifteenth letter of the Greek alphabet, its name means "small o," and it looks the same as the Latin letter o.

What is the difference between omicron and omega?

Omicron (ο) means "small o" and was originally the short o sound; omega (ω) means "big o" and was the long o sound. Omega is the last letter of the alphabet and a common math symbol, while omicron is the fifteenth letter and rarely used in formulas.

Is the O in big-O notation an omicron?

Historically it is a Latin O, chosen by Bachmann and Landau for "Ordnung." Knuth proposed reading it as a capital omicron in 1976 to pair it with Ω and Θ. In practice you type it as a Latin O, and in LaTeX as O(n) or \mathcal{O}(n).

How do you type the omicron symbol?

In Word type 03BF and press Alt + X. On a Mac or phone, add the Greek keyboard and press O. In HTML write ο. In LaTeX there is no command, so write o or O. Or copy ο from the box at the top of this page.

Why was the COVID variant called Omicron?

The WHO named variants of concern with consecutive Greek letters. When B.1.1.529 was classified in November 2021, the next letters were nu and xi; the WHO skipped nu because it sounds like "new" and xi because it is a common surname, so the variant became Omicron.

Writing with Greek Symbols

Greek text, star catalogues and algorithm analysis each need the right "o," typeset consistently and never mixed up with a zero. Octree is an AI-powered LaTeX editor that turns plain-language descriptions into properly formatted equations. Create professional documents at useoctree.com.