How does an Oxygen Mask work?

The oxygen flowing into the mask flushes the exhaled breath containing the CO2. This is the reason a mask should not be run flow rates below about 5 liters per minute. Since the mask does not fit skin tight, most of the air escapes around the edges. The holes are mainly a safety feature to allow some opening for inhalation should the mask somehow create a seal around the edges, and to allow excess flow to escape.