Open o or turned c (majuscule: Ɔ, minuscule: ɔ) is a letter of the extended Latin alphabet. In the International Phonetic Alphabet, it represents the open-mid back rounded vowel. It is used in the orthographies of many African languages using the African reference alphabet.

The Yucatec Maya language used Ɔ to transcribe the alveolar ejective affricate [t͡sʼ] consonant in the orthography of the Colonial period. Now dz or tsʼ is preferred.

On the macOS US Extended keyboard, ɔ and Ɔ can be typed with ⌥ Option+: followed by c or C.^{[citation needed]}

Open o looks like a reversed letter 'C'. Claudius introduced a Ɔ (the antisigma) with the intention of replacing bs and ps.

The Scandinavian explanatory symbol (forklaringstegnet) can be typeset using the open o followed by a colon, thus: ɔ:. It is used to mean "namely", "id est", "scilicet" or similar.

This letter is often used to refer to the Copyleft official sign, which looks like an open o with a circle around it.