In December of last year, I ended up having all my teeth extracted and dentures made. I had immediate dentures, so there were three dental appointments to make impressions, etc., and I had to have my mouth open for several minutes. I took my supplemental oxygen with me, and the dentists were well aware of my condition. Fortunately, I had no difficulty, but those were not long sessions.
However, when I had my teeth extracted, the process took an hour and a half. To be on the safe side, the extraction was performed at the VA hospital, and I used their O2, and had the nose hose attached for the entire time. My mouth was definitely open for an extended time; but I breathe through my nose. I had my own pulse-oximeter, and I checked it regularly. The oral surgeon and the assistants kept asking me if I was OK and if I needed a break.
I had no problems at all.