New Rescue Divers often ask about whether it is legal to give a patient oxygen in an emergency, and whether it might cause medical complications. These are valid concerns, but within the scope of diving, administering oxygen in an emergency isn't really an issue

in most areas, there are no laws prohibiting buying medical oxygen for emergency use, or administer­ing oxygen in an emergency Some areas stipulate that the individual be trained in oxygen adminis­tration (PADI Rescue Diver and/or other emer­gency oxygen diver certifications quality within the scope of dive emergencies) As long as the patient consents, in most countries there's nothing illegal about providing oxygen in a dive emergency (if the patient is unconscious content is implied).

Only a few countries prohibit giving oxygen in an emergency. 
It has been thought that giving oxygen can make a few medical conditions worse, but theres some doubt about this now, Nonetheless, these conditions include emphysema and other lung diseases that impair individuals significantly. People suffering from these are not candidates for diving. Healthy individuals can suffer lung irritation if they breathe high oxygen concentration to long, but this takes hours - more than likely youll have the patient in professional medical care, or run out of ­emergency oxygen first. Therefore, according to DAN recommendations and current emergency care protocols, you dot have to worry about making someone worse by administering oxygen in a dive emergency