O-mouthed - You're so taken aback you look like a pigeon that just saw a

You're so taken aback you look like a pigeon that just saw a flamingo wearing a crown.

My dad told me he's moving to Australia. I was O-mouthed.

I saw my neighbor’s kid riding a dinosaur. I was O-mouthed.

My dog started singing opera. I was O-mouthed.