KenV wrote:

Technically, the pilot's oxygen connection is attached to the ejection seat and the ejection seat get its oxygen from the jet.  In the event of an ejection, the seat is equipped with a small oxygen bottle good for 5 to 10 minutes and it automatically switches to the internal bottle when you pull the ejection handle.  The 5 to 10 minutes is long enough to survive the ejection at high altitude till you drop down to a low enough altitude to breath on your own.  If you land in the water, you can actually breath underwater for a short time till you come to the surface.  That's why the post ejection procedures over water are different than overland.  Overland you want to loosen you oxygen mask before ground impact in case you're knocked unconsious.  If you're unconcious when your seat oxygen goes empty, your dead.  But when you eject over water you cinch up your oxygen mask straps to make sure it has a water tight seal around your face.  You can breath underwater till you come to the surface.  Once at the surface, you loosen your oxgen mask or dump it entirely.

In the F-16 things are a little different. The hose from your mask is connected to a connector on your harness and that connector is in turn connected to the aircraft and also the emergency bottle on your seat.
Also, our ejection procedures are the same over land and water concerning the mask. We get rid of it !. If you are unconcious when you land in water and you haven't removed your mask it is automatically removed after contact with water. Your risers are also automatically disconnected and your vest is inflated. You get your boat already while hanging in the chute.
I guess they figured if you get knocked unconcious at ejection there is no garantie you are going to wake up before the oxygen is used.