heart, interior
奧 centers on the deep, innermost part of something: the heart of a matter or the interior of a space. It points to what is hidden, central, or profound.
奧 is the traditional form of 奥. It combines 宀 (roof) with 釆 (distinguish) and 大 (big), suggesting a deep, secluded interior space within a building. The modern simplified form 奥 drops one stroke.
Under a roof (宀), a large (大) hidden space holds the heart of the matter. Picture a vast, dim interior where the deepest secrets are kept.
For オウ, imagine the deep interior of an old house echoing with a long 'Ohhh' sound: Oh -> オウ, as you discover the heart of the building.