If you jumped into a pool of liquid oxygen, would your body instantly crystallize?
Nothing happens instantly. The first thing would be frostbite to the skin followed by the onset of hypothermia to the internal organs. No doubt everything would "freeze up" with time. What this really brings up though is safety issues with cryogenic fluids, that is, those substances that are normally gases (like oxygen, nitrogen, carbon dioxide, hydrogen, or helium) at room temperature but can be changed to liquid form through the use of very low temperatures.
Besides the usual danger of handling anything that cold, there's something much more dangerous with these liquids. If they're exposed to the much warmer conditions in which we live, they will convert very quickly back into gas form. The thing is that they will expand out to hundreds(!) of times their liquid volume. In some situations, this can mean that the air (and oxygen) around you will be displaced by this gas. If it contains no oxygen then you may end up breathing an atmosphere almost devoid of any oxygen. The worst thing about this is that you will NOT realize that you are not breathing proper air and you will pass out within seconds. Extreme injury and death can result all too quickly.
In the case of oxygen, you also have another danger. If the liquid oxygen is converting back into a gas, then you will have too much oxygen around you. Oxygen is what is needed to keep a fire burning. So with extra oxygen around, substances that are normally almost non-flammable can suddenly be very flammable! If you want to learn more, you can try out this NASA web site.
Carl Zorn, Detector Scientist (Other answers by Carl Zorn)