curving shoreline, bend in river
澳 centers on a curved or indented shoreline or riverbank: a place where water bends inward, forming a cove or inlet.
澳 combines 氵 (water) with 奧, which likely contributes the sound and the idea of a deep or recessed place, together suggesting a deep bend in a body of water.
Water 氵 beside 奧, a shape associated with depth and interior space, suggests a deep, curving inlet where water bends into the land.
For オウ, imagine an old curved shoreline where you say 'Oh!' in wonder at the bend: Oh -> オウ.
Macao (China); Macau