plaster over with layer of mud
圬 centers on applying a coating of mud or plaster to a surface, like plastering a wall.
圬 combines 土 (earth, mud) with 亏, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side is 土 (earth, mud) and the right side 亏 looks like a trowel or a hand spreading something. Picture a worker using mud to plaster a wall.
For オ, imagine an old plasterer saying 'Oh!' as he slaps mud onto the wall: Oh -> オ.