2Na + 2H2O --> 2NaOH + H2
Sodium reacts with water to produce NaOH and hydrogen gas.
You could calculate the pressure produced by a certain amount of Na using PV=nRT equation for ideal gas.
Also the hydrogen produced is flammable so could be burned in oxygen as a fuel.
Maybe you could produce the hydrogen first, then draw this into the cylinder with oxygen gas and ignite them, what do you think?