Without the mask, one is breathing cabin air anyway. With the mask, one is breathing cabin air with a higher percentage of oxygen, and that's the point.
In fume/smoke events there isn't a problem with the amount of O2 in the air, so adding a bit doesn't help any. (That is why they only help in situations with a lack of O2 - i.e at altitude.) In fact, the lung doesn't use all of the oxygen available at normal (below 10,000 feet) pressure, and expires the amount not used - which is why EAR (Expired Air Resuscitation, "mouth to mouth") works.
The problem is the combustion poisons present - which the mask doesn't alleviate . As before - the O2 masks available in the cabin don't help in smoke/fume events, and may indeed hinder by convincing a user without proper education that they do help.