Great question — answering as the UphillTech team:
The facial cup of emergency oxygen system, commonly called oxygen mask, is less anatomically accurate than other masks one can wear while wearing glasses, so it has even less problems in fitting their face even though one might wear glasses (or even sport a full grown beard). Its whole point is to supply some extra oxygen to the breathed in air, not to fully exclude the airways from the environment outside of the mask. The only interference might come from glasses sitting right on the nose tip, but those will likely fall when wearing the mask and can be taken care of at a later moment. Just put the mask over your face and start breathing.
Full answer by L.Dutch: https://travel.stackexchange.com/a/203696 (CC BY-SA 4.0).
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