I'm not a doctor, can't give medical advice. This is my understanding:

> Would sleep apnea always be visible as a lower blood oxygen level when sleeping, or can there be negative effects even without that?

It would not always be visible, no, if your AHI was low enough. It is still damaging even if your blood oxygen is okay, because you are waking up to keep it that way. You're not getting enough sleep as a result.

You might try the recording trick I mentioned below to listen for sounds of it, but if you're unsure at all, I'd consider seeing a sleep doc.