O2 cleaned and O2 service rated cylinders will maintain their rating if they are filled only with:

Oxygen-compatible gas refers to a mixture or gas that is specifically designed to prevent combustion or degradation when in contact with high concentrations of oxygen. When O2 cleaned and O2 service-rated cylinders are filled with oxygen-compatible gas, they can maintain their rating because these gases are formulated to ensure safety and compatibility with the cylinder's internal environment. This is crucial because the presence of non-compatible gases could risk combustion or degrade the cylinder's integrity, especially under high pressures typically used in diving applications.

Filling these cylinders with nitrogen, any breathing gas, or air does not guarantee the same level of safety. Nitrogen may not react with oxygen under normal circumstances, but it doesn't mean it is safe for high partial pressures of oxygen. Any breathing gas could contain various elements that might compromise the integrity of oxygen service-rated equipment, depending on the gas mix. Air, which is composed of approximately 21% oxygen, may not be suitable for all applications at elevated pressures where the risk of oxygen toxicity or combustion increases. Thus, maintaining the rating of the cylinder is contingent upon using only oxygen-compatible gases.