Liquid Oxygen Canister
The canister can contain a maximum of 1 bucket (1000 mB) of Liquid Oxygen (also known as LOX). Vanilla buckets and other liquid receptacles cannot be used to hold Liquid Oxygen, as due to its cryogenic (extremely cold) temperature, it needs special handling in a canister.
The red bar icon drawn on the side of the canister shows approximately how full it is; there is also a tooltip which states the contents exactly.
If you have other mods installed with fluid-handling capabilities (for example, Buildcraft, IC2 or Railcraft) it's also possible to pipe Liquid Oxygen out of the Gas Liquefier machine into tanks, and then fill Canisters of Liquid Oxygen from those tanks in the same way that you will normally fill a bucket.
The main use of Liquid Oxygen is as a handy way to transport oxygen.
One full Canister of Liquid Oxygen (1000 mB) is the equivalent of 5,555 units of breatheable oxygen, that's enough to fill 4 Heavy Oxygen Tanks. The Canister of Liquid Oxygen can fill those tanks simply by crafting it together with a tank in a crafting inventory.
Canisters of Liquid Oxygen can also be used as a portable oxygen source for machines which use oxygen, for example the Oxygen Distributor. This can be a great way to get a simple space base started, if you are in a hurry!
It is possible more uses of Liquid Oxygen will be added in future versions of Galacticraft. There is also discussion on the forum about whether, for realism, Liquid Oxygen (LOX) should be a requirement for rockets, in addition to fuel. The mod authors have not yet taken any decision on that.
Use in a crafting grid with a Light Oxygen Tank, Medium Oxygen Tank or Heavy Oxygen Tank, and the Canister of Liquid Oxygen will fill the tank if it can. The crafting result will be the filled tank, and the depleted Canister of Liquid Oxygen will be returned to your inventory.