While oxygen itself is not flammable, oxygen, especially pure oxygen, increases the flammability of other substances, especially in confined spaces where the oxygen ratio can increase above normal levels. An atmosphere containing more than 23.5% oxygen is defined as “oxygen-enriched.” It requires less fuel and less heat for fire to break out in oxygen-enriched atmospheres. You should take special precautions to reduce the fire hazard in oxygen-enriched environments, as all organic materials and even some traditionally noncombustible substances (like metal pipes) can catch fire.
At the other end of the spectrum are oxygen-deficient atmospheres, which contain less than 19.5% oxygen. These places fall under the confined space regulations. In essence, any confined space where oxygen levels can dip below 19.5% is hazardous to life or health. For such spaces, a written permit, special training for employees who may enter into the confined spaces, and precautions, whether it be appropriate personal protective equipment, a buddy system, or special communications procedures, are all appropriate.