-o may refer to:

• the -o affix found in English and many other languages.
• Macron

Barred O (majuscule: , minuscule: ) is a letter in several Latin-derived alphabets.

O is one of the 12 Vietnamese language vowels. It is pronounced (an unrounded [o]).

O (named o , plural oes) is the fifteenth letter and a vowel in the ISO basic Latin alphabet. The letter was derived from the Semitic `Ayin (eye), 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 similar Egyptian hieroglyph for "eye". The Greeks are thought to have come up with the innovation of vowel characters, and lacking a pharyngeal consonant, employed this letter as the Greek O to represent the vowel , a sound it maintained in Etruscan and Latin. In Greek, a variation of the form later came to distinguish this long sound (Omega, meaning "large O") from the short o (Omicron, meaning "small o").

(o-grave) is a letter in the Kashubian language. This letter also appears in Catalan, Italian, Lombard, Occitan, Scottish Gaelic, Taos, and Vietnamese language as a variant of letter “o”.

(o-acute) is a letter in the Faroese, Hungarian, Icelandic, Kashubian, Polish, Czech, Slovak, and Sorbian languages. This letter also appears in the Catalan, Irish, Occitan, Portuguese, Spanish, Italian and Vietnamese languages as a variant of letter “o”. It is also used in English for other purposes. This also appears in Pumpokol.

Not to be confused with O, O with double acute.

"Ö", or "ö", is a character used in several extended Latin alphabets, or the letter O with umlaut to denote the front vowels or . In languages without umlaut, the character is also used as a "O with diaeresis" to denote a syllable break, wherein its pronunciation remains an unmodified .

Ø or (minuscule) ø is a vowel and a letter used in the Danish, Faroese, Norwegian and Southern Sami languages. It is mostly used as a representation of mid front rounded vowels, such as ø œ, except for Southern Sami where it's used as an [oe] diphthong.

In mathematics, big O notation is used to describe the limiting behavior of a function when the argument tends towards a particular value or infinity, usually in terms of simpler functions. It is a member of a larger family of notations that is called Landau notation, Bachmann–Landau notation (after Edmund Landau and Paul Bachmann), or asymptotic notation. In computer science, big O notation is used to classify algorithms by how they respond (e.g., in their processing time or working space requirements) to changes in input size.

A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system. Some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens can stem from one allele (or very closely linked genes) and collectively form a blood group system. Blood types are inherited and represent contributions from both parents. A total of 30 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT).

A breve (; ; from the Latin brevis "short, brief") is a diacritical mark ˘, shaped like the bottom half of a circle. It resembles the caron (i.e. wedge or háček in Czech), but is rounded, while the caron has a sharp tip. Compare Ǎ ǎ Ě ě Ǐ ǐ Ǒ ǒ Ǔ ǔ (caron) with Ă ă Ĕ ĕ Ĭ ĭ Ŏ ŏ Ŭ ŭ (breve).

A caron ( ˇ ) or háček (; from Czech háček ), also known as a wedge, inverted circumflex, inverted hat, is a diacritic placed over certain letters to indicate present or historical palatalization, iotation, or postalveolar pronunciation in the orthography of some Baltic, Slavic, Finno-Lappic, and other languages. The caron also indicates the third tone (falling and then rising) in the Pinyin romanization of Mandarin Chinese.

The circumflex ( ˆ ) is a diacritic used in the written forms of many languages, and is also commonly used in various romanization and transcription schemes. It received its English name from Latin circumflexus (bent around)—a translation of the Greek περισπωμένη (perispōménē). The character is also used in mathematics, where it is typically called hat or roof.

The double acute accent ( ˝ ) is a diacritic mark of the Latin script. It is used primarily in written Hungarian, and consequently is sometimes referred to as Hungarumlaut, a portmanteau of Hungarian umlaut. The signs formed with diacritic marks are letters in their own right in the Hungarian alphabet (for instance, they are separate letters for the purpose of collation).

An emoticon is a pictorial representation of a facial expression using punctuation marks and letters, usually written to express a person's mood. Emoticons are often used to alert a responder to the tenor or temper of a statement, and can change and improve interpretation of plain text; emoticons for a smiley face :-) and sad face :-( appear in the first documented use in digital form. The word is a portmanteau word of the English words emotion and icon. In web forums,  instant messengers and online games, text emoticons are often automatically replaced with small corresponding images, which came to be called emoticons as well. Certain complex character combinations can only be accomplished in a double-byte language, giving rise to especially complex forms, sometimes known by their romanized Japanese name of kaomoji.

In mathematics, and more specifically set theory, the empty set is the unique set having no elements; its size or cardinality (count of elements in a set) is zero. Some axiomatic set theories assure that the empty set exists by including an axiom of empty set; in other theories, its existence can be deduced. Many possible properties of sets are trivially true for the empty set.

This is a list of notable and commonly used emoticons or textual portrayals of a writer's mood or facial expression in the form of icons. The Western use of emoticons is quite different from Eastern usage, and Internet forums, such as 2channel, typically, show expressions in their own ways. In recent times, graphic representations, both static and animated, have taken the place of traditional emoticons in the form of icons.

A macron, from the Greek  (makrón), meaning "long", is a diacritic placed above a vowel (and, more rarely, under or above a consonant). It was originally used to mark a long or heavy syllable in Greco-Roman metrics, but now marks a long vowel. In the International Phonetic Alphabet the macron is used to indicate mid tone; the sign for a long vowel is a modified triangular colon ⟨⟩.

An object file is a file containing relocatable format machine code that is usually not directly executable. Object files are produced by an assembler, compiler, or other language translator, and used as input to the linker.

The ogonek (Polish , "little tail", the diminutive of ogon; Lithuanian nosine) is a diacritic hook placed under the lower right corner of a vowel in the Latin alphabet used in several European and Native American languages.

Oneworld (CRS: *O), branded as "oneworld", is one of the world's three largest global airline alliances with its central management team, Oneworld Management Company, based in New York City, New York, USA. Oneworld was founded in 1999 by American Airlines, British Airways, Canadian Airlines, Cathay Pacific, and Qantas. The alliance slogan is "Oneworld revolves around you".

Oneworld may refer to:

• Oneworld, an international airline alliance
• Oneworld Publications, a British independent publishing firm founded in 1986 by Novin Doostdar and Juliet Mabey
• OneWorld.net, a nonprofit web portal about sustainable development and human rights issues around the globe
• OneWorldTV  a nonprofit video-sharing website dedicated to showcase documentaries about developing countries
• Institute for OneWorld Health, a nonprofit pharmeceutical company
• OneWorld, a computer software suite developed by JD Edwards, a company now owned by Oracle Corporation

Shift Out (SO) and Shift In (SI) are ASCII control characters 14 and 15, respectively (0xE and 0xF).  The original meaning of those characters was to switch to a different character set and back.  This was used, for instance, in the Russian character set known as KOI7, where SO starts printing Russian letters, and SI starts printing Latin letters again. SO/SI control characters also are used to display VT-100 pseudographics, and emoji (Japanese picture icons) on SoftBank Mobile. ISO/IEC 2022 standard specifies their generalized usage.