Oxygen is a medication and must be prescribed by your doctor. Doctors use several tests to determine if you need home oxygen and if so, how much oxygen.
Chest X-rays determine the condition of your lungs and if you have lung disease.
Blood tests determine how well your lungs are working and they confirm how much oxygen your blood transports to your body.
Exercise tests determine how well your body uses the oxygen it receives.
Respiratory assessments determine the flowrate required to maintain adequate oxygen levels at rest and during exercise.
Based on these tests, your doctor will write a prescription for home oxygen therapy stating two things:
a specific flowrate of oxygen, usually measured in litres per minute (lpm)
how many hours per day you must use oxygen (requirements often depend upon activity level), so your doctor may also order a higher oxygen flowrate to use when you are active