jar, jug, vat
甕 centers on a large, sturdy container for holding liquids or bulk goods: a jar, jug, or vat. The idea is always a capacious vessel, often made of earthenware.
甕 combines 雍 (likely a phonetic component) with 瓦 (earthenware), indicating a type of pottery vessel. The exact historical development of the upper component is uncertain, but the character consistently refers to a large jar or vat.
The bottom 瓦 is a tile or earthenware, and the top 雍 looks like a complex lid or stopper. Picture a heavy clay jar with an elaborate sealed top, used for storing large amounts of liquid.
For オウ, imagine an enormous jar so big you say 'Oh!' when you see it: 'Oh!' -> オウ, and the jar is so large it could hold a vat of water.