Take the oxygen to your local hospital. Clinical Engineering or Biomedical Technology departments are responsible for seeing that ALL hospital equipment is properly calibrated, and should have access to O2 monitors. Respiratory therapy departments use O2 monitors a lot. If you are in a very rural area your paramedics should have an O2 meter. University chemistry/biology departments should have O2 meters, too.
Final local calibration of these meters can be done with a Mass Spectrometer. Anesthesia gas monitors use a Mass Spec. Usually a 2 point calibration test with pure gas without oxygen for 0%, and room air for 21%. Known calibration gasses with specified % chemical compositions are also used.
When I used to get cylinders filled for SCUBA purposes the "medical grade" cylinders were all in the same line with the welding gas cylinders. Only difference was med grade had chrome platted brass valves.