This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
歍 combines 烏 (crow) and 欠 (open mouth, lack). The exact historical development is uncertain.
A crow (烏) with an open mouth (欠) might suggest a cawing sound, but the meaning is unclear.
For オ, imagine a crow opening its mouth and letting out a surprised 'Oh!': Oh -> オ.