lid of a cauldron, Buddhist cloister
盦 centers on a covering or enclosure: a lid for a cauldron, or by extension a cloistered space such as a Buddhist retreat. The two meanings connect through the idea of something that covers or contains.
盦 combines 酓 (a phonetic component) with 皿 (dish/vessel), suggesting a vessel-related meaning. The exact historical development is uncertain, but the structure points to a lid or covering for a container.
The top part 酓 looks like a cover, and the bottom 皿 is a dish. Picture a lid placed over a cauldron: 盦 is that lid. The cloister meaning extends the idea of a covering into a sheltered space.
For オウ, imagine an old cauldron with a lid that makes a deep 'oh' sound when you lift it: 'oh' -> オウ, and the lid covers the vessel.