to wrap and bind, damp, book bag
裛 centers on enclosing something in cloth: wrapping and binding an object, or a damp cloth used for wrapping. The book bag sense is a specific application of a cloth wrapper.
裛 combines 衣 (clothing/cloth) with 邑, which likely contributes a phonetic element. The character expresses wrapping or binding with cloth, and the damp sense may relate to moistening cloth for wrapping.
Picture a cloth garment 衣 wrapped tightly around a small town 邑, binding it like a damp book bag.
For オウ, imagine an old book bag wrapped in cloth, and you say 'Oh!' when you see how neatly it's bound: Oh -> オウ.