An unintended combustion event occurred in the Rocky Mountain region during the transfilling of four compressed oxygen cylinders on a cart in an aviation hangar, injuring the employee performing the operation. The cylinders were being filled from a separate source compressed oxygen cylinder that had been recently connected to the cart manifold. The incident was captured by a surveillance camera. The video shows a bright flash erupting from the oxygen cart and the fireball enveloping the employee's face, expanding upward, receding to the cart, and extinguishing. The flash originated from a stainless steel ball valve that had been installed, along with a stainless steel cross fitting, by a new outside contractor about two months prior to the incident. The objective of this investigation was to determine what led to the combustion event. The investigation process included examining the aviation hangar; examining the source cylinder, cart cylinders, and cart manifold; analyzing samples taken from various locations within the oxygen system for oils and particulates; and performing cleanliness calculations. During the investigation, it was discovered that the new stainless steel valve and cross fitting ordered by the outside contractor were not cleaned for oxygen service. Ultimately, it was concluded that the hydrocarbon- and silicone-based oils and greases associated with the lubricants and manufacturing process of the new stainless steel valve were the main fuels that probably initiated the kindling chain reactions. Once the oils and greases were ignited, other fuels involved in the kindling chain resulting in burnout included the valve lubricants, valve packing, and metals associated with the stainless steel valve body, side rings and discs, ball stem, gland, springs, and packing bolt. The ignition mechanism was most likely related to flow friction, compression heating, or resonance.