Why do you possibly want to use a polycarbonate container, can't metal pressure tanks hold the stuff just fine? You do know LN2 is only cold when it is allowed to expand. Holding up the container to the CPU won't make it cold at all.
As for liquid oxygen, I don't think you can even get your hands on it since it is highly reactive. Can rust metals extremely quickly. Also highly explosive. Oxygen in such concentration and pressure is in danger of reacting with hydrogen in the air potentially casuing an explosion. Also breating excess oxygen is bad for your body, it can oxidize the metals within your body. Divers and hospitals never use pure oxygen, but a mix.
Polycarbonate is not an ideal material for a pressure tank. If you want something virtually indestructable and can hold a much greater PSI, look into carbon fiber tanks. Though they are more expensive.