* Gases are compressible: Unlike liquids, gases can be compressed into a smaller volume. So, even if the pressure gauge on the tank reads half its full pressure, the tank isn't necessarily half full of gas molecules. It just means the pressure inside the tank is half the maximum pressure it can hold.
* Pressure and volume are related: The amount of gas in the tank is related to its pressure and volume. If the pressure drops by half, the volume of gas inside the tank would also be half its original volume, but the tank itself is still the same size.
Think of it this way: Imagine a balloon. When it's full, it has a certain amount of air inside. You can let some air out, making the balloon smaller. But the balloon itself is still the same size, even though there's less air inside.
So, instead of "half empty," it's more accurate to say the oxygen tank has half its original pressure or that it has used half of its initial gas volume.
