The FAA is cautioning cabin and cockpit crewmembers to guard against passengers attempting to bring personal oxygen bottles on board an aircraft. Contained O2 is considered a hazardous material both as a non-flammable gas and an oxidizer.
Part 121 and 135 regulations do allow air carriers to provide passengers with onboard compressed oxygen for personal use, provided they follow guidelines in their FAA-approved aircraft maintenance manual. However, these carriers are not allowed to permit passengers to bring their own oxygen tanks aboard.
Some air ambulance flights can be permitted to carry passenger oxygen bottles aboard for use at the destination, provided the operator follows guidelines in CFR 175.501 (e).