Consistent oxygen saturation of 89-90 while sleeping. is this considered obstructive sleep apnea?

Right on the brink: Typically, O2 sats while asleep in a healthy individual should be >90. If yours are typically hitting 90 or below then, I'd say you are right in the crack between barely normal and barely abnormal. I'd think for a 19 year old nonsmoker you could do a bit better than that so it all depends on what else is going on. That number by itself doesn't diagnose OSA but "yes" you could have it.

Suggestions: There are many causes of lower oxygen saturation during sleep. OSA is one of them. I would suggest noting down all of your symptoms, including daytime drowsiness, taking the Epworth test online, and following up with your physician. If there is snoring more 4 or more times per week, there may be sleep disordered breathing.

Sleep study...: There are many possible reasons for your oxygen saturation to be low during sleep. A sleep study would diagnose what is going on with your breathing during sleep and monitor, not only oxygen sat, but also brain waves, respiratory muscle activity, heart, etc. This test would diagnose the reason for low oxygen levels with obstructive sleep apnea being one of many possibilities. See your doctor

Perhaps: Perhaps. Other reasons for low oxygen. See your MD. Sleep studies may be indicated.