The authors investigated a fire in an industrial oxygen cylinder filling system that resulted in ignition and combustion of a steel compressed gas cylinder. The incident occurred when the operator found the subject cylinder valve closed (cylinder had not been filled) and subsequently opened the cylinder valve when there was a high pressure differential between the manifold and the cylinder. Based on the physical evidence, a seal in the residual pressure valve (RPV) installed at the cylinder valve outlet was the most probable fire origin and the first material that ignited. Other observed damage to the cylinder valve and cylinder was consistent with a kindling chain developing from ignition of the RPV seal during high gas flow into the cylinder from the filling manifold. Significant combustion damage to the valve threaded connection and associated region of the cylinder was observed. Notably, a sintered filter element installed at the cylinder valve stem visually appeared to be constructed of copper with a sintered bronze element. However, analysis of the filter indicated that the threaded portion was a steel alloy (copper-coated). The flammability of this component contributed to propagating the fire to the cylinder. Observations and evidence indicated that combustion occurred internal to the gaseous oxygen (GOX) cylinder until the pressure containment was breached when the structure in the cylinder neck area failed due to thinning and heating. Observed damage to the fill system flex hose and fill adapter fitting was consistent with external damage after breach of the cylinder. Several operational and design issues contributed to the ignition and fire event.