Denise, I don't have COPD or asthma, either, but my oxygen desaturated to 52% during my sleep study. I do have sleep apnea, but I had a lot more hypopnea episodes. Hypopnea means "underbreathing" (less than 50% of capacity), and you can do it for extended periods, and this is what usually leads to very low oxygen numbers. Unlike apnea episodes, hypopneas won't usually wake you up (which is a bad thing, because at least then you would breathe).
In my case, the problem is anatomy--my lower jaw and upper airway are just smaller than my body requires. I always thought my overbite was just a cosmetic thing. . . not, apparently. My son has the same problem, even though his overbite was corrected in his teens (fixing the bite doesn't change the size of your airway, duh).
Supplemental oxygen therapy is standard in cases like yours (pending the sleep study). The good news is that it is likely only temporary because the real problem can be corrected with a dental appliance or a cpap machine. I think the oxygen is a cya for the doctors. . . .all the oxygen in the world doesn't help if it can't get to where it's needed.
I've been on cpap for 2 1/2 years (LOVE IT!)--but our power went out during the blizzard and I was without my cpap for the first time. Most miserable night ever.
Gentle hugs, Denise, and hang in there. You are on your way to better sleep and more energy, I promise!