32 grammes O2 occupies 22.4 litres at STP, i.e. zero deg C, and one atmosphere pressure.
(P1 * V1) / T1 = (P2 * V2) / T2
Where P = pressure in atmospheres, V = volume, T = Temperature in Degrees Absolute = deg C + 273.
Crudely, this means for example, if you double the pressure, you double the mass of gas in a given volume, or for a given mass, you halve the volume, providing the temperature stays constant.
Not being used to American barrels, I don't know how many litres there are in a barrel. But at least you now have the basics