Barred o (capital: , lowercase: ) is a letter in several Latin alphabets.

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

O (named o , plural oes) is the 15th letter and the second-to-last vowel in the modern English alphabet and the ISO basic Latin alphabet.

-o may refer to:

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

(o-grave) is a letter in French, Catalan, Lombard, Occitan, Kashubian, Scottish Gaelic, Taos, Vietnamese, Haitian Creole, and Welsh. It also appears in Italian as a variant of 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 Galician languages as a variant of letter “o”. It is sometimes also used in English for loanwords.

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 an "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, Norwegian, Faroese and Southern Sami languages and in Old Swedish. It is mostly used as a representation of mid front rounded vowels, such as [ø] and [œ], except for Southern Sami where it is used as an [oe] diphthong.

In mathematics, big O notation describes 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. In analytic number theory, it is used to estimate the "error committed" while replacing the asymptotic size, or asymptotic mean size, of an arithmetical function, by the value, or mean value, it takes at a large finite argument. A famous example is the problem of estimating the remainder term in the prime number theorem.

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 an alternative version of a gene) and collectively form a blood group system. Blood types are inherited and represent contributions from both parents. A total of 35 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT). The two most important ones are ABO and the RhD antigen; they determine someone's blood type (A, B, AB and O, with +, - or Null denoting RhD status).

A breve (, less often ; ; from the Latin brevis “short, brief”) is the diacritic mark ˘, shaped like the bottom half of a circle. As used in Ancient Greek, it is also called vrachy or brachy. It resembles the caron (the wedge or háček in Czech) but is rounded; the caron has a sharp tip.

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

The circumflex is a diacritic in the Latin, Greek and Cyrillic scripts that is used in the written forms of many languages and in various romanization and transcription schemes. It received its English name from Latin circumflexus "bent around"a translation of the Greek περισπωμένη (perispōménē). The circumflex in the Latin script is chevron-shaped ( ˆ ), while the Greek circumflex may be displayed either like a tilde ( ˜ ) or like an inverted breve (   ̑ ).

Diethyl ether, also known as ethoxyethane, ethyl ether, sulfuric ether, or simply ether, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid. It is commonly used as a solvent and was once used as a general anesthetic. It has narcotic properties and has been known to cause temporary dependence, the only symptom of which is the will to consume more, sometimes referred to as etheromania.

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 by typographers as Hungarumlaut. 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 ( or ), etymologically a portmanteau of emotion and icon, is a metacommunicative pictorial representation of a facial expression that, in the absence of body language and prosody, serves to draw a receiver's attention to the tenor or temper of a sender's nominal non-verbal communication, changing and improving its interpretation. It expresses — usually by means of punctuation marks (though it can include numbers and letters) — a person's feelings or mood, though as emoticons have become more popular, some devices have provided stylized pictures that do not use punctuation.

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 ensure 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 moods or facial expressions 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. These are commonly known as emoji although the term kaomoji is more correct.

A macron is a diacritical mark, a straight bar placed above a letter, usually a vowel. Its name derives from the Greek  (makrón), meaning "long", and was originally used to mark long or heavy syllables in Greco-Roman metrics. It now more often marks a long vowel. In the International Phonetic Alphabet, the macron is used to indicate a mid-tone; the sign for a long vowel is instead a modified triangular colon .

O is the fifteenth letter of the modern Latin alphabet.

An object file is a file containing object code, meaning relocatable format machine code that is usually not directly executable. There are various formats for object files, and the same object code can be packaged in different object files. An object file also works like an Application Extension (.dll).

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

Oneworld (marketed as oneworld; CRS: *O) is an airline alliance founded on 1 February 1999. The alliance's stated objective is to be the first-choice airline alliance for the world's frequent international travelers. Its central alliance office is currently based in New York, New York, in the United States. Its member airlines include Air Berlin, American Airlines, British Airways, Cathay Pacific, Finnair, Iberia, Japan Airlines, LAN Airlines, Malaysia Airlines, Qantas, Qatar Airways, Royal Jordanian, S7 Airlines, SriLankan Airlines and TAM Airlines, plus some 30 affiliated airlines. As of 31 March 2014, Oneworld is the third largest global alliance in terms of passengers with 512.8 million passengers, behind Star Alliance (637.6 M) and SkyTeam (588 M). Its slogan is "An alliance of the world's leading airlines working as one."

Oneworld is an international airline alliance.

Shift Out (SO) and Shift In (SI) are ASCII control characters 14 and 15, respectively (0x0E and 0x0F.) These are sometimes also called "Control-N" and "Control-O".

Ø (Disambiguation) is the seventh studio album by American metalcore band Underoath. Released on November 9, 2010, through Tooth & Nail Records, the album was the band's only without founding member Aaron Gillespie, and is the first and only record by the band with Daniel Davison, formerly of Norma Jean. It was also their final album before a two year breakup from 2013 to 2015. Ø (Disambiguation) was met with a high amount of acclaim and was recorded at Glow in the Dark Studios in Atlanta, Georgia, the same studio where the band's previous album, Lost in the Sound of Separation was recorded.

Ø (and ø) is a Scandinavian vowel letter.