Can an oxygen canister ever be half empty?

In real life, no.

A gas always expands to fill whatever container you put it in.  As you
use up the oxygen in the canister, the oxygen that's left expands and
fills the whole canister.  So the canister is always completely full, but
the pressure of the gas inside keeps going down as you take the
gas molecules out of it.

A SCUBA divers may often say that his tank is "half empty", or that
he only has "a quarter of a tank left".  He actually means that he has
used up half of the oxygen in his tank, by staying down for half of the
time that the tank is good for.  The gas that's still  inside his tank is
still filling up the whole inside.


Now, you WILL hear people say that there IS a chance that the gas
in the tank CAN all go over into one half of the tank and leave the other
half empty.  Technically, that's true.  The gas molecules are all zipping
and zapping around inside the tank, at different speeds and in different
directions.  Once in a while, two of them collide and go off in new directions
at new speeds.  This is all random, and there's no way to predict what any
one molecule or group of them will do.  We can only tell the probability (the
chances) of the average speed and direction of a huge number of them. 

When we do that, we find that there is a small chance that they'll all be
in the same half of the tank for a short time.  There's also a small chance
that all the air molecules in your bedroom will be in the same half of the
room for a short time, and there won't be any air to breathe over on YOUR
side. 

This is true.  Technically.  BUT ... the probability (the chance) is SO SMALL
that scientists who work with gases don't even put it into their equations. 
We just assume that the probability of this happening is zero ... there's no
chance.  It's that small.  And real gases in the real world go along with this
assumption very well.

Ice melting no wait thats global warming

Answerseasonal dry and wet winds this answer is right outta of the book