DOT regulations (see CFR 49) strictly prohibit over-pressurizing a scuba cylinder containing any kind of gas, but it is particularly dangerous to do so with high oxygen concentrations. The reason is: The higher the pressure and the higher the oxygen concentration, the higher the risk of a fire and explosion if a contaminant is present.
Luxfer has received numerous and persistent reports that technicians in certain sectors of the recreational diving community routinely over-pressurize scuba cylinders, including cylinders containing high concentrations of oxygen. This is sometimes described as “doing divers a favor,” offering “a little more down time” or “giving divers their money’s worth.” Not only is this an unsafe practice, it is against the law! Under no circumstances should you allow the gas pressure in your scuba cylinder to exceed the service pressure for which the cylinder is designed and stamped or marked. If a filler offers to over-pressurize your cylinder, you should not only refuse the offer, you should report the filler to the DOT. If you suspect that your cylinder has been over-pressurized, you should have it depressurized and have it inspected by a competent technician to determine whether it is fit for further service.
For pure oxygen, DOT mandates strict pressure limits: Gas pressure in an aluminum cylinder containing pure oxygen must never exceed 3,000 psi (even if the cylinder is stamped for a pressure above 3,000 psi).
If you are concerned about running out of oxygen, use a larger decompression cylinder filled to the proper service pressure—or carry more cylinders.