Did you ask your doctor if using a CPAP machine would help?
An oxygen saturation level of 90% is low, but I think not low enough to require any appliances including a CPAP. Do you sleep on your back, try sleeping on your side.
What is your age, are you over weight... lots of factors you doctor knows but I can only guess.
I had periods of low oxygen in the range of 85% but I got enough REM sleep to be rested (I had an in-hospital sleep study done). I lost about 10% of my body weight and the symptoms of Obstructive Sleep Apnea have gone, I have not been retested, and may request an overnight oximeter test to see what it shows on my O2 saturation levels. Oh yes I am also old,, that doesn't help either as all the muscles go soft including those in the throat, which seems to have been my problem.