Checking and preparing an emergency oxygen system for return to service includes all of the following EXCEPT:

Keeping the regulator pressurized as part of routine checks for an emergency oxygen system is not necessary when preparing it for return to service. In most cases, regulators should not be left pressurized unless the system is actively in use or being prepared for emergency deployment. Ensuring that the oxygen system is in a safe and serviceable condition is critical, which includes inspecting the cylinder for any damage, checking the functionality of the flow meter, and making sure that all connections are secure.

When the system is not in use, a regulator should ideally not be pressurized to avoid unnecessary wear or potential failure of components. Hence, it is good practice to relieve pressure in the system when it's not actively being utilized, ensuring the longevity and reliability of the equipment for future emergencies.