The Vulcan B Mk2 strategic (nuclear) bomber had a Rover Gas Turbine Airborne Auxiliary Power Unit (AAPU) In flight it was started by firing a pyrotechnic starter cartridge through the compressor to spin it up, then discharging oxygen directly into the compressor while applying fuel and ignition. The oxygen charging point and the oil filler cap were co-located under the same access panel, with the two charging points being about four inches apart. The Vulcan B Mk2 remained in service for more than thirty years and despite the AAPU oil and oxygen being replenished on every pre-flight and daily check, not once did we ever have an oxygen fire or explosion due to this arrangement.
As I said earlier, 'Much ado about nothing'. Oxygen charging on the line or in the hangar is perfectly safe as long as you follow the laid down procedures.