I am a mask fetishist in general. That is my main thing. As a preteen or so, I used to be turned on by scenes of a girl being given the anesthesia mask, which was usually black, and the oxygen mask, which was clear. As I've grown older, I certainly don't mind those scenes/moments, but my main fetish is when the female patient needs to wear a protective mask. Say, a disposable N95, a Respro, a yellow earloop, or a Vogmask.
One thing, that isn't really traditionally medfet, but is nonetheless a lifesaving measure, are those yellow passenger masks on airliners, that drop down from above. I saw "Final Descent" recently, an old airline disaster movie from 1997, and they had a few pretty girls wearing those.
What I kind of like is seeing the girl masking herself up rather than somebody else doing it for her. Something about her knowing it's best for her, and the same can apply to the oxygen masks discussed as the OP's topic.
Some people are discussing coverage area of the mask. I only like masks that cover the nose and mouth both. Not just the nose (mouth is fine, if the mask slips a bit).
And I don't like wearing them. It's somehow sexy on a girl but embarrassing on me.