aterpster: Not all airlines use cabin warning horns, installed or not
Alber Ratman: for passengers, the oxygen is mixed with cabin air in the oxy mask (steady flow w/bottles, (quickly) decreasing flow w/generators), don't use with smoke in the cabin.
Flight deck masks are set do 100%, initially, but should and can be set to mix with ambient air (steady pressure), to save oxygen, e.g. for prolongued crossing of high terrain.