answer, yes, OK, reply, accept
應 centers on responding to something: giving an answer, replying, accepting, or saying yes. The idea is a reaction that matches or meets what is presented.
應 is the traditional form of 応. It combines 心 (heart/mind) with a phonetic component historically derived from 䧹, which contributed the sound and the idea of a bird responding. The modern simplified form 応 replaces the complex upper part with 广.
The outer shape 䧹 suggests a bird, and the inner 心 is the heart. Imagine a trained bird that answers your call with a chirp, responding from its heart.
For オウ, picture the bird answering with a loud 'Oh!' sound: Oh -> オウ, as it replies to your call.