An EMS helo was consumed in a fire while parked at a hospital heliport. The helo had recently returned from a medevac flight and the on-board patient oxygen pressure was low. Two respiratory therapists participated in refilling the system after the flight. The system is serviced via a rear baggage compartment, accessed from outside the aircraft. The attendants indicated that after the connection was made and the valve was partially opened, a fire erupted near the connection. Subsequent to the fire, one of the therapists remembered the presence of oil above the baggage door. This area of oil was not wiped clean prior to commencing the fill operation. Subsequent to the fire, the valve was tested and significant leaking at the connection was present. After the test a wrench was needed to separate the components for inspection. The inspection provided no evidence of debris or distortion of the threaded portion of the connection. There was no intent for flight in this particular operation and no flight crew was present.