1. Care must be taken to prevent cylinder top valve assemblies snapping off due to mechanical shock like falls. An oxygen cylinder has 2250 PSI pressure inside of it, and broken cylinders have become missiles and gone through thick hospital walls, and killed people.
2. If you remember the demonstrations in high school chemistry pure 100% oxygen is a fire accelerant: things (like cigarettes) will burst into flame, small sparks (like in the Apollo astronaut disaster) can light huge flash fires. Things burn pretty well in a regular atmosphere of 21% oxygen - think of how things could burn in 100% oxygen.
2. Other gases like nitrogen, helium, etc. released in quantity can displace the oxygen in a closed room causing human asphyxiation.