jar, jug, container
瓮 centers on a container for holding liquids or goods: a jar, jug, or similar vessel.
瓮 combines 公 (which likely contributes a sound element) with 瓦 (earthenware), indicating a clay jar or container. The exact historical development of the top component is uncertain.
The top 公 looks like a lid or opening, and the bottom 瓦 is a clay vessel. Picture a clay jar with a public lid: 瓮 is a container for everyone to use.
For オウ, imagine an old jar labeled with a big 'O' on it: 'O' -> オウ, and the jar holds water.