Risks of oxygen
Pure oxygen under pressure is so reactive that it will react violently with many solid materials (including cylinder wall), provided an adequate source of ignition energy is given.
That energy maybe as small as a shock, a spark from a steel wrench, or the oxidation of tiny amounts of grease (including fingerprints on exposed parts).
Oxygen filling procedures are stringent and should be followed very carefully. Care and cleanness (sp?) are essential.