Ǫ
O with an ogonek (majuscule: Ǫ, minuscule: ǫ) is a letter of the Latin alphabet formed by the addition of the ogonek (from Polish: little tail) to the letter O. It is used in Western Apache, Mescalero-Chiricahua, Muscogee, Dadibi, Gwichʼin, Erie, and Navajo. It is also used in the Latin transcription of Old Church Slavonic, and the Proto-Slavic language, as well as in the Slavistic Phonetic Alphabet. It is also still in use for the writing of Old Norse, and used to be used sporadically in Polish. Usage 
The letter is used in the autochthonic languages of North America: Western Apache, Mescalero-Chiricahua, Muscogee, Dadibi, Gwichʼin, Erie, and Navajo. In such languages, it represents either a nasalized close-mid back rounded vowel ([õ]), or a nasalized ([ɔ̃]). It is also used in the Latin transcription of Old Church Slavonic where it represents the nasal back vowel, as well as in the Proto-Slavic language where it represents a labialized non-front vowel. It is also used in the Slavistic Phonetic Alphabet, where it represents the nasalized O-sound, for example, the pronunciation of Ą in Polish. It was used in Old Norse, where it represented the open back rounded vowel ([ɒ]) sound. Additionally, the letter sporadically used to be an alternative to Ą in Polish. Encoding

References 

Latin letters with diacritics
Phonetic transcription symbols
Polish letters with diacritics