Will you die from inhaling someone elses' oxygen?

When people exhale they are not giving out oxygen but are putting out extra carbon dioxide which is poisonous to humans (that's why we blow it out). But the amount of oxygen we remove from the air we inhale, and the amount of carbon dioxide we add to our exhaled air, is not nearly enough to be immediately dangerous to someone else. This is why you can revive a person by carefully breathing into their mouth while pinching their nose, as in rescue breathing.

The only way exhaled air becomes hazardous to yourself or others is if you are exhaling for an extended time in a small, air tight space. Then you use up enough of the oxygen, and add enough carbon dioxide, to create a dangerously low oxygen concentration, or a dangerously high carbon dioxide concentration.