It is true that the Occupational Safety and Health Administration requires employers to keep their environment’s air at 19.5% oxygen or higher. But wearing face masks will not cause serious health effects, and they prevent the wearer from spreading the virus to others.
The COVID-19 virus alone is smaller than the N95 filter size. But the virus travels attached to larger particles consistently caught by the filter, and regardless of size, the erratic motion of particles — and the electrostatic attraction generated by the mask — means viruses get consistently caught, too.
There is no evidence that the general public will experience oxygen reduction significant enough to result in hypoxemia. Carbon dioxide can build up in face masks, but is unlikely that wearing a mask will cause hypercapnia, according to the CDC.