an echo, sound, to vomit (as an infant)
咉 centers on a sound that comes back or is produced from within: an echo, a vocal sound, or the involuntary sound of an infant vomiting.
咉 combines 口 (mouth) with 央, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to sounds from the mouth.
A mouth 口 beside 央, like a central opening producing a sound that echoes back. Imagine a baby's mouth making a small sound that comes back as an echo, or the sound of vomiting.
For オウ, picture an echo bouncing back from a mountain: 'Oh!' you shout, and the echo returns 'Oh!' — Oh -> オウ.