- Not to be confused with Ő, O with double acute.
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The HTML entity is Õ for Õ and õ for õ.
In Estonian, Õ is the 24th letter of the alphabet (between V and Ä), representing the close-mid back unrounded vowel sound [ɤ]. The vowel had previously been marked with 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 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.
Mathematical use 
Encoding problems 
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.
See also 
Letter O with diacritics
Letters using tilde sign ( ◌̃ )