The Canadian Underwater Mine-countermeasures Apparatus (CUMA) is a self-contained, semi-closed circuit underwater breathing apparatus used by the Canadian Forces for underwater mine search, investigation, and disposal. On November 30, 2001, the Experimental Diving Unit staff at Defence Research and Development Canada — Toronto was preparing for an experimental dive using the CUMA Version 2 (V2) in their facility's hyperbaric chamber. A fire occurred in the CUMA V2 as the Team Leader (diver wearing the CUMA V2) opened the oxygen sphere valve. The fire was severe, ejecting fire and molten metal for a distance of approximately 7 feet from the diver's back-mounted unit, and lasting for an extended duration before the diver's teammates were able to remove the backpack and extinguish the fire. No one was seriously injured in the incident. Wendell Hull & Associates, Inc. (WHA) was contracted to investigate the oxygen fire, specifically to perform a cause and origin analysis, and to provide recommendations for the CUMA V2 oxygen system to improve its ignition resistance and fire tolerance. WHA reviewed the incident details, system components, materials of construction, and operational procedures and conditions. WHA also inspected the burned hardware and disassembled individual components as required to characterize the burn and melt-flow patterns. Key materials were sampled as required and chemical analyses on samples were done to obtain positive material identification. The evidence is consistent with the local origin of the fire being within the first-stage regulator close to the non-metal seat. The evidence also indicates that operationally induced ignition mechanisms and incompatible materials were causative factors in the ignition and propagation of the fire. This paper describes the events of the oxygen fire incident, the protocol used to analyze the fire's cause and origin, and the results of the analysis. The recommendations to improve CUMA V2's ignition resistance and fire tolerance are also discussed.