Because Taiwanese is a tonal language the standard letter without a diacritic represents the vowel in the first tone, the other four possible tone categories require one of the following tonal symbols to be written above it.
- Ó͘ ó͘ (second tone)
- Ò͘ ò͘ (third tone)
- Ô͘ ô͘ (fifth tone)
- Ǒ͘ ǒ͘ (sixth tone)
- Ō͘ ō͘ (seventh tone)
The character was introduced by the Xiamen-based missionary Elihu Doty in the mid-nineteenth century, as a way to distinguish the Minnan vowels /o/ and /ɔ/ (the latter becoming ⟨o͘⟩). Since then it has become established in the Peh-oe-ji orthography, with only occasional deviations early in its usage – one example being Carstairs Douglas's 1873 dictionary, where he replaced the ⟨o͘⟩ with an o with a curl (similar to that of the English Phonotypic Alphabet), and a second example being Tan Siew Imm's 2016 dictionary of Penang Hokkien, where she replaced the ⟨o͘⟩ with ⟨ɵ⟩.
- Klöter, Henning. "The History of Peh-oe-ji" (PDF). Archived from the original (PDF) on 2011-05-29.
- Douglas, Carstairs (1990) . Chinese English Dictionary of the Vernacular or Spoken of Amoy. Taipei: Southern Materials Center. ISBN 957-9482-32-2.
- Tan, Siew Imm (2016). Penang Hokkien-English Dictionary. Areca Books. Retrieved 16 October 2018.
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