long necked jar or bottle
罃 refers to a long-necked jar or bottle, a specific type of container.
罃 combines 缶 (jar) with a phonetic element above, likely indicating a type of vessel. The exact historical development is uncertain.
The bottom component 缶 means jar or can. The upper part looks like a lid and a long neck, so picture a jar with a tall narrow neck.
For オウ, imagine an old long-necked jar labeled with the sound 'O' (like the letter O) and 'U' (you) written on it: O-U -> オウ.