You need to have a test done at the doctor's office. You may then be referred for a breathing functions test. Oxygen is a prescription-required substance, Depending on how your numbers are, your doctor may or may not prescribe supplemental oxygen for you

I did that test about a month ago from another doctor and now I am getting ready to start taking trelegy 200 for me . No o2 yet thank God also am on muncenex and fluticasone propionate nasal spray and boy has it helped me @ almasummer and @ michaelzerjav . Good luck with your things. I do know when I have a hard time breathing I relax and do slow breathing sitting in a chair or lying down on the floor while listening to some soft music from my phone and it does work and there are some days I even fall asleep

That is a hard one to call. The doctor has a 6 minuet, 300 foot walk test they will have you do to see if your O2 percentage drops below 87%.

For me, it was about 4 years after being diagnosed that I realized I was having troubles walking from the car to the shopping carts at the grocery store. I brought it up at my next pulmonary appointment and he had me do the test I only walked up and down the hall way twice and he said that was enough. I was put on oxygen while walking or doing anything strenuous.
Hope that helps you or at least gives you a reason to make an appointment with your pulmonologist to discuss it.