island
嶴 centers on an island, specifically a mountainous one rising from the sea.
嶴 combines 山 (mountain) with 奧 (deep, interior), suggesting a deep mountain rising from the sea as an island. The exact historical development is uncertain.
A deep mountain (奧) rising from the sea with a mountain base (山) forms an island: 嶴 is a mountainous island.
For オウ, imagine an old man saying 'Oh!' as he spots the mountainous island: Oh! -> オウ.