content: I don't think dry chemical extinguishers displace oxygen, but rather, they stop the fire by acting as a flame retardant, stopping the chemical processes of "fire". CO2 extinguishers, on the other hand, do work by displacing the oxygen, which the combustion process needs to continue burning.  Same thing with halon extinguisher systems, I think.  We had halon systems in CATVs in the Army that worked by instantly displacing all the oxygen within the vehicle.