"Õ" (uppercase), or "õ" (lowercase) is a composition of the Latin letter O with the diacritic mark tilde.

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The HTML entity is Õ for Õ and õ for õ.

For Romagnol language, õ is used in some proposed orthographies to represent [õː], e.g. savõ [saˈvõː] "soap". To this day a unified standardization has not been established.

In Estonian, Õ is the 27th letter of the alphabet (between W and Ä), and it represents a vowel characteristic of Estonian, the unrounded back vowel /ɤ/, which may be close-mid back, close back, or close-mid central.^[1] The vowel was previously written with the letter Ö, but in the early 19th century, Otto Wilhelm Masing adopted the letter Õ, ending the confusion between several homographs and clearly showing how to pronounce a word.

In informal writing, e.g., emails, instant messaging and when using foreign keyboard layouts where the letter Õ is not available, some Estonians use the characters O or 6 to approximate this letter.

In most parts of the island Saaremaa, Õ is pronounced the same as Ö.

In the Guarani language, Õ is the 22nd letter and fourth nasal vowel of the alphabet, similar to the Spanish "o", but with a stressed nasalization.

In Hungarian, Õ only appears when a typeface (font set) does not contain a proper ő letter, which is an o with a double acute diacritic. The letter Õ is not part of the Hungarian alphabet, it is an error of improper computer font sets.

In Samogitian the letter Õ represents, as in Estonian, the unrounded back vowel /ɤ/ which is unique to Samogitian and is not found in Standard Lithuanian, this is a rather new innovation brought on by the ensuing efforts of standardising Samogitian, this letter alleviates the confusion between the two distinct pronunciations of the letter ė.

In the Portuguese language, the symbol Õ stands for a nasal close-mid back rounded vowel, also written [õ] in IPA. It is not considered an independent letter of the alphabet: the tilde is the standard diacritic for nasalization.

In the Vietnamese language, the symbol Õ stands for the sound [ɔ] with creaky voice (rising tone with a glottal break followed by a continuation of the rising tone). Vietnamese also has derived letters Ỗ/ỗ and Ỡ/ỡ.

In the Võro language, this letter is the 25th letter of the alphabet, pronounced as in Estonian.^[2]

In the Skolt Sami language, this letter is the 25th letter of the alphabet, pronounced as [ɘ].

In the Voko language, the letter Õ represents 'ɔ̀ŋ'.

The symbol, pronounced soft-O, is used as a variant of big O notation to measure growth rate that ignores logarithmic factors.^[3] Thus, ${\displaystyle f(n)\in {\tilde {O}}(g(n))}$ is shorthand for ${\displaystyle \exists k:f(n)\in O(g(n)\log ^{k}n)}$.^[3]

Due to character encoding confusion, the letters can be seen on many incorrectly coded Hungarian web pages, representing Ő/ő (letter O with double acute accent). This can happen due to said characters sharing a code point in the ISO 8859-1 and 8859-2 character sets, as well as the Windows-1252 and Windows-1250 character sets, and the web site designer forgetting to set the correct code page. Õ is not part of the Hungarian alphabet. The usage of Unicode avoids this type of problems. In Latex the option of using "\~o" and "\~O" exists.

• Ã
• Tilde