hollow in the ground, cavity, undulating
坳 centers on a depression in the earth: a hollow, cavity, or undulating ground. The idea is a low-lying area or uneven terrain.
坳 combines 土 (earth) with 幼, which likely contributes the sound and suggests something small or immature, possibly referring to a small depression in the ground.
The left side is earth 土, and the right side 幼 looks like a small, immature shape. Picture a tiny hollow in the ground, like a child's footprint in the soil.
For オウ, imagine an old hollow in the ground that echoes when you shout 'Oh!' into it: Oh -> オウ, and the sound bounces around the cavity.