rice, seedlings
秧 centers on young rice plants: the seedlings that are grown before being transplanted into paddies. The meaning extends naturally to rice itself as the crop these seedlings become.
秧 combines 禾 (grain) with 央, which likely contributes the sound and may suggest the central or young stage of the plant. The character is used for rice seedlings.
The grain radical 禾 beside 央 looks like a young plant at its center stage. Picture a rice seedling standing tall in the middle of a nursery bed.
For オウ, imagine an old farmer saying 'Oh!' as he sees the rice seedlings sprout: Oh -> オウ, and the grain 禾 is growing strong.