O͘o͘ is one of the six Hokkien vowels as written in the Pe̍h-ōe-jī (POJ) orthography. It is pronounced [ɔ]. Because Hokkien is a tonal language, the standard letter without a diacritic represents the vowel in the first and fourth tone with the fourth and eighth tone always only used in syllables with a syllable stop (i.e. ⟨-p⟩, ⟨-t⟩, ⟨-k⟩, ⟨-h⟩ /-ʔ/), and the other six to eight possible tone categories require one of the following tonal symbols to be written above it:

• Ó͘ ó͘ (second tone) 《陰上／阴上》
• Ò͘ ò͘ (third tone) 《陰去／阴去》
• Ô͘ ô͘ (fifth tone) 《陽平／阳平》
• Ǒ͘ ǒ͘ (sixth tone, used in Quanzhou-descended dialects) 《陽上／阳上》
• Ō͘ ō͘ (seventh tone) 《陽去／阳去》
• O̍͘ o̍͘ (eighth tone) 《陽入／阳入》
• Ŏ͘ ŏ͘ / Ő͘ ő͘ (ninth tone, high rising in Taiwanese Hokkien)

The character was introduced by the Xiamen-based missionary Elihu Doty in the mid-nineteenth century, as a way to distinguish the Hokkien vowels /o/ and /ɔ/ (the former becoming ⟨o͘⟩).^[1] Since then it has become established in the Pe̍h-ōe-jī orthography, with only occasional deviations early in its usage – one example being Carstairs Douglas's 1873 Chinese–English Dictionary of the Vernacular or Spoken Language of Amoy, where he replaced the ⟨o͘ ⟩ with ⟨ө̛ ⟩ (an o with a curl, similar to that of the English Phonotypic Alphabet),^[2] and a second example being Tan Siew Imm's 2016 dictionary of Penang Hokkien, where she replaced the ⟨o͘ ⟩ with ⟨ɵ⟩.^[3]

In the Unicode computer encoding, it is a normal Latin o followed by U+0358 ◌͘ COMBINING DOT ABOVE RIGHT, and is not to be confused with the Vietnamese Ơ. This letter is not well-supported by fonts and is often typed as either o· (using the interpunct), o• (using the bullet), o' (using the apostrophe), oo (as used in Tâi-lô for Taiwanese Hokkien and Wāpuro rōmaji for Japanese), or ou (as used in Wāpuro rōmaji for Japanese).