"Medical grade oxygen is considered any concentration of oxygen above 85%. Oxygen concentrators will provide anywhere from 90% to 95% pure oxygen while running properly."

Oxygen (as O2 gas) is a very strong molecule. It takes a lot of energy to break it down for good use in the body. Or like in a car engine.
Plants can use the Sun power to create new oxygen from Carbon diOxide, our breathing enemy. Then that Oxygen can come together with Hydrogen to make some Water - H2O - our very best friend. But our damaged lungs are so poor and slow at exchanging the old CO2 for some fresh O2 in the air sacs damaged by Emphysema and bronchitis. That's the Obstruction part of COPD.

That information is on the specifications of the individual machines but normally its 95% + or- 5 percent and I think that most machines alarm at 80%.

Most units produce somewhere between 90 and 95%.