sing
哦 centers on vocal expression through singing, with the mouth radical 口 emphasizing the act of producing song.
哦 combines 口 (mouth) with 我, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to vocal sound.
The mouth 口 beside 我 suggests a person using their mouth to sing: 哦 is the act of singing.
For ガ, imagine a singer belting out a note with a strong 'ga' sound: ga -> ガ, and the mouth 口 is wide open in song.
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