Question

Can a tank of oxygen gas ever be half empty?

Answer 1

No. Any amount of gas always expands to completely fill
whatever container it's in.

So, as you take oxygen out of the tank, the pressure of what's
left in the tank certainly becomes less and less, but whatever is
left in the tank always expands and fills the whole tank. 

(And then, eventually, when the pressure inside the tank drops
to equal the pressure outside it (atmospheric pressure), you could
cut off the whole top of the tank and nothing more would come out.)

SCUBA divers regularly talk about having a 'full tank', and 'half a tank',
and maybe a 'quarter tank'.  But that refers to the amount of time that
the tank can still deliver air to them under water.  It's not the amount
of volume inside the tank that's full or empty.  The inside of the tank
is always full. 

Think about it this way:  ==>  There's no way to take half of the air
out of a balloon, leaving it half empty and still inflated. Doesn't happen.

Answer 2

Yes depending on the rate of oxygen intake. Also, the amount of pressure may limit the amount of air in a tank.