Although OBOGs have been around for over half a century, only in the last two decades have OBOGs become compact, cheap, and reliable enough to replace the older compressed gases or LOX (liquid oxygen) as a source of breathable air for high flying aircrew. Aircraft have been staying in the air longer (because of in-flight refueling) and carrying enough compressed oxygen has become untenable, so it has become necessary for OBOG to replace it. Since the 1990s, most American military aircraft have replaced older oxygen systems with OBOG. Most Western nations, and now Russia, have followed, at least with their latest model aircraft.
Most OBOG systems work by using a chemical reaction to remove nitrogen from the air taken in to the OBOG, and then sending out air with the proper amount of oxygen to the aircrew.