apply, answer, yes, OK, reply, accept
応 centers on a responsive action: applying oneself to a situation, answering a call, or accepting a request. The meanings 'yes' and 'OK' are natural affirmative responses.
応 is the modern simplified form of 應. The older form combines 心 (heart/mind) with a phonetic component that also suggests a bird's response, conveying the idea of answering or reacting from the heart.
Under a roof (广), the heart (心) responds: when someone calls, your heart answers, so 応 means to reply, accept, or apply yourself.
For オウ, imagine an 'O' shape forming around your heart as you answer 'OK': O -> オウ, and your heart responds.
correspondence (to); equivalence
reaction; response
aid; assistance; help; support; reinforcement
more or less; though not quite satisfactorily; after a fashion; pretty much; roughly; so far as it goes; technically
application; subscription; entry (competition, raffle, etc.); enlistment
to answer; to respond; to meet
(practical) application; putting to practical use
appropriate; adequate; suitable; fitting; worthy
adaptation; accommodation; conformity
reply; answer; response
suitable; appropriate; befitting; becoming; adequate; reasonable; proper
to respond; to answer; to meet (e.g. demands, expectations)
to respond; to satisfy; to accept; to comply with; to apply for
emergency fix; tentative measure; temporary solution
adaptation; accommodation; conforming; adjustment
dealing with (people, customers, complaints, etc.); receiving (callers, visitors, etc.); attending to; handling; serving
feeling in the hands (when striking, poking, etc.); (feeling of) resistance
hailing each other
exchange (of words, views, punches, cups, etc.); give-and-take
unsuited; inappropriate; improper; undeserved
reception (e.g. of visitors); dealing with
making a bid; tendering a bid
side effect (of a vaccine)
according to one's abilities; appropriate; reasonable