The location of the Medical Gas shutoff valves should be known in your area. The process should follow something similar to this. However, sites containing medical oxygen should all have a detailed risk assessment and fire safety training for all users as to what to do.

• Nurse in Charge will assess the situation from the staff
  to decide if the ward is to be evacuated therefore switch
  off, or stay in place and leave on.
• If turning off – ensure portable cylinders are
  available as required.
• Remove patients to a place of safety or assembly
  point.
• If you have evacuated without turning them off, please
  inform Estates who can do this safely. Alternatively, inform
  the Fire Brigade when they arrive.