There are no contraindications for giving oxygen therapy. If a person is having a stroke or cardiac problems, then they MUST have oxygen. Their problem is with the body not receiving oxygen-carrying blood in both situations. If a person has COPD (chronic obstructive pulmonary disease), such as emphysema, they do not have the properly functioning biological sensors to determine the proper oxygen/co2 blood levels, so if they have too much oxygen, they might stop breathing. Firstly, they shouldn't be diving if they have copd. Secondly, if they stop breathing, you just breath for them. They will start breathing again. The only other problem is at the surface, if a person breathes 100% oxygen for several DAYS, they might develop a dry respiratory system or pulmonary oxygen toxicity. Of course, at depth, depending upon the oxygen concentration in the breathing gas, neurological toxicity is possible. That is why nitrox is not for diving deeper. you can't hurt any diver by giving them oxygen, even if they don't need it.