nightingale, bush warbler
鴬 centers on a specific small songbird: the Japanese bush warbler, often called a nightingale in English contexts. The meaning is tightly tied to this bird rather than a broad conceptual cluster.
鴬 is the modern simplified form of 鶯. The older form combines 鳥 (bird) with a component that likely contributed the sound and possibly the idea of a small bird. The modern top portion is a simplification.
The top ⺌ and 冖 look like a small nest or shelter, and the bottom 鳥 is a bird. Picture a tiny bush warbler nesting under a protective cover.
For オウ, imagine the bush warbler singing a clear 'oh' note: 'oh' -> オウ, and the bird's song echoes from the nest.