Before servicing a high-pressure oxygen cylinder, it must be:

Before servicing a high-pressure oxygen cylinder, it is essential to inspect it for external damage. This is crucial because any visible damage, such as dents, rust, or signs of corrosion, could compromise the integrity of the cylinder and pose a safety risk when it is under high pressure. Ensuring that the cylinder is in good condition helps prevent leaks or catastrophic failures during operation, which can lead to hazardous situations.

While the other choices involve important safety and maintenance practices, they are not the immediate priority before servicing. For example, hydrostatic testing is required periodically but does not need to be performed before every service event unless the cylinder shows signs of damage. Repainting for safety markings is a good practice but does not affect the immediate need to ensure the cylinder's structural integrity. Purging with inert gas is also a safety measure, typically performed during specific operational procedures rather than as a precautionary step before servicing. Thus, the most critical first step is to inspect for external damage.