Question

Why is oxygen gas usually stored in metal cylinders?

Answer 1

The reason oxygen and other gasses are stored in metal cylinders is that of the simple fact it's a gas.  The reasoning behind it is to safely store the gas with no leaking etc.

"The purpose of this safety policy and procedure is to establish guidelines for the protection and safety of ([COMPANY]) employees who handle and use compressed gases. Compressed gases are typically stored under pressure in metal cylinders. These cylinders are designed and constructed to withstand high pressures. Improper handling and use of compressed gases can result in devastating consequences.

This safety policy and procedure provide guidelines for the safe handling and use of compressed gases. It includes provisions for training and presents safe handling guidelines. It also presents the types, uses, inspection, and marking requirements of compressed gas cylinders. Additionally, this safety policy and procedure present transportation and storage requirements for compressed gas cylinders."

quoted answers belong to: 

https://www.safetyinfo.com/written-safety-programs-compressed-gas-cylinder-safety-program-free-index...

I take NO CREDIT in this answer.  All quoted answers are copied from the link above,  I am only here to help, not to get into trouble.  

Answer 2

Federal and state guidelines offer regulations related to the storage of oxygen cylinders, which state that the cylinders should be racked and kept away from combustible agents. Oxygen cylinders are used in industrial, educational, medical and consumer areas and should always be handled with care.

Oxygen cylinders are hazardous because of the pressure inside them. They should always be stored in an upright and secure position since a fall could damage the tank, could cause a rupture, or could cause the tank to become an airborne hazard itself. The oxygen inside the cylinder may be very cold, particularly if the gas is to be used for cryogenic or freezing purposes. Such gases cause skin and other materials to become very brittle and can result in injuries that are similar to thermal burns.Oxygen cylinders must be stored away from flammable sources as their contents could ignite. Oxygen should also not be stored in the same space with other tanks of flammable gases or liquids. Additionally, federal and state regulations provide clear guidelines for how many tanks may be stored in a single area. Individual tanks for medicinal purposes present similar hazards and should be handled in according to the same precautions.Oxygen tanks should be stored in rooms designated for the purpose. The room should have sufficient area to move the cylinders and should be secured with locking doors. The room must meet appropriate fire ratings, ventilation requirements and construction codes. The racks used to secure both filled and empty tanks must meet similar code requirements.