In the United States, the best way to answer the question is with the truth. If a person is ok, he should say he is ok. If he is not ok, he should say that as well.
The United States is known for putting an impersonal face on emotions and emotional relationships. Because of this, it can be very confusing for people from other cultures to understand how to respond to such questions as "How are you" and "Are you ok?" Fortunately, the latter tends to be a genuine expression of concern for the person asked, rather than a rote conversational gambit. This makes it socially acceptable to answer the question with the truth, rather than a stock answer.