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.
Did you ask your doctor if using a CPAP machine would help?