Can Apple Watch detect obstructive sleep apnea?

No. It is not designed to do that. But in my experience, the Apple Watch Sleep feature, used together with the AutoSleep app, provides good information that I share with my doctor. It provides a graph of how many times I am awake and in different sleep stages. For me, it shows that the CPAP is keeping my sleep apnea in check.

It’s an expensive solution, but the Oura Ring (https://ouraring.com/) has an option to continuously measure blood oxygen levels.

Great answer!

Thanks, IdrisSeaBright!