[4] Middle Armenian is notable for being the first written form of Armenian to display Western-type voicing qualities and to have introduced the letters օ and ֆ, which was based on the Greek letters "o" and "φ".
It represents the IPA sound /ↄ/ (open-mid back rounded vowel).
Although this is a native phoneme in Armenian, the sound that would usually represent it, ո, was used only in medial and final positions.
It represents the IPA sound /f/ (voiceless labiodental fricative), which is not a native phoneme in Armenian.
However, it was prevalent in borrowed words, making it necessary to use a letter to write it.