Is it true that a medical device (portable oxygen concentrator) is exempt from the carryon baggage allowance?

Can you give me a link showing (online) the actual regulation? I looked at the IATA website and could not find it. When I say exempt from the carry on bag allowance, I mean that it is my understanding that a person who needs an oxygen concentrator can carry the machine PLUS one carry one bag PLUS a personal bag (e.g. briefcase, purse, etc). But I wanted to see the actual regulation and download it. The person is flying from the US to Asia on Singapore Airlines. Thanks.

Its up to the person at the gate, but there wouldn't usually be a problem. If you have concerns, you can call customer service of the airline and ask, they can put a comment in your booking to say its allowed. If you haven't already, make sure its ok to take the oxygen on board, because some airlines wont allow it on without prior notice, because it classified as dangerous goods.