wild duck, easy mark
鴨 centers on a wild duck, and by extension an easy mark or sitting duck—someone easily taken advantage of.
鴨 combines 甲 (which may represent a helmet or hard shell, possibly alluding to the duck's head or bill) with 鳥 (bird) to denote a duck. The exact historical origin of the 甲 component in this context is uncertain.
The left side 甲 looks like a helmet or a target, and the right side 鳥 is a bird. Picture a wild duck wearing a little helmet, making it an easy mark for hunters.
For オウ, imagine a wild duck letting out a loud 'Oh!' as it realizes it's an easy mark: 'Oh!' -> オウ.
duck
lintel (esp. one with grooves for sliding doors)
spot-billed duck (Anas poecilorhyncha)
mallard (Anas platyrhynchos); mallard duck
cross between a mallard and domestic duck
desirable prey; easy game; easy mark; easy prey; easy target
gadwall (Anas strepera)
northern shoveler (species of duck, Anas clypeata); northern shoveller
Fuegian steamer duck (Tachyeres pteneres); Magellanic flightless steamer duck
green-winged teal (species of duck, Anas crecca)
northern pintail (Anas acuta)
Baikal teal (species of dabbling duck, Anas formosa)
harlequin duck (Clangula hyemalis or Histrionicus histrionicus)
Auckland teal (Anas aucklandica); Auckland Islands teal
long-tailed duck (Clangula hyemalis); old-squaw; hareld; old injun; oldwife
common shelduck (Tadorna tadorna)
Eurasian wigeon (Anas penelope)
any duck with white specula (bright patches on the wings)
Falkland steamer duck (Tachyeres brachypterus)
falcated teal (Mareca falcata); falcated duck