martingale, breast harness, saddle girth, fetter, shackle, carry on back
鞅 centers on a leather strap used to control or secure: a martingale or breast harness for a horse, a saddle girth, or by extension a fetter or shackle. The idea of carrying on the back also fits the harness theme.
鞅 combines 革 (leather) with 央, which likely contributes the sound. The character originally referred to a leather strap used in horse tack, such as a martingale or breast harness.
Leather 革 plus a central 央 shape: imagine a central leather strap running down a horse's chest, like a martingale, keeping everything secure.
For オウ, picture a horse wearing a leather harness and saying 'Oh!' as it's tightened: Oh -> オウ.