Õ

Emilian-Romagnol

In Emilian-Romagnol, õ is used to represent [õː], e.g. savõ [saˈvõː] "soap".

Estonian

In Estonian, Õ is the 27th letter of the alphabet (between W and Ä), used to represent a vowel characteristic of Estonian, the unrounded back vowel /ɤ/, which may be mid back, close back, or mid central. 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 of Saaremaa Island, Õ is pronounced the same as Ö.

Portuguese

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.

Vietnamese

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.

Võro

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

Skolt Sami

In the Skolt Sami language, this letter is the 25th letter of the alphabet, pronounced quite like in Estonian and Võro.

Mathematical use

The symbol, pronounced soft-O, is used as a variant of big O notation that ignores logarithmic factors. Thus, f ( n ) ∈ O ~ ( g ( n ) ) is shorthand for ∃ k : f ( n ) ∈ O ( g ( n ) log k ⁡ g ( n ) ) .

Computer encoding

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.