great drought, heat
熰 centers on intense, destructive heat: a great drought that scorches the land, or extreme heat in general.
熰 combines 火 (fire) with 區, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with heat and drought.
Fire 火 on the left and the complex shape 區 on the right suggest a vast, oppressive heat. Imagine a fire so intense it creates a great drought, baking everything under a relentless sun.
For オウ, picture a great drought where the heat is so intense you can only say 'Ow!' as you touch the scorched ground: Ow -> オウ.