New CPAP user here too. I've noticed something similar. Before CPAP, my Fitbit regularly gave me scores in the 80s. Very rarely, I would score above 90. With CPAP, my scores have become much more variable: I'm getting a lot more scores above 90 and a lot more scores below 80.My subjective sense is that the quality of sleep I'm getting is quite good (I feel great), though I do notice I'm sleeping less (~7 hours real-time and about 6 hours on Oura/Fitibit with lower sleep scores [75]), but I feel pretty good so I'm not that concerned about the Oura scores.
The reason for the lower scores is that I am "short" sleeping a lot more. Atypically for me, I often wake up around 5:00ish and can't go back to sleep. My theory about this--and it is just a theory--is that my sleep has become more consolidated and efficient, so that I have less "sleep pressure" if I happen to be misfortunate enough to wake up an hour or so early.
The reason for my higher scores primarily is the result of higher restoration scores. Both my Fitbit and my oximeter (when I decide to wear it) are reporting consistently lower heart rates at night. My heart rates were not particularly bad before, but I am seeing much more time spent under 60 beats per minutes than before.
I haven't noticed any real dips in my heart rate, though my resting HR is about 50 and at night I typically dip between 43 - 45 BPM. Will keep an eye on this and will be better about wearing my O2 ring (so many devices!)