A cylinder with s capacity of 4.0L is filled with oxygen to a pressure of 1500kPa. How much oxygen, measured at standard pressure, does this cylinder contain?

You mean standard temperature?

If so, then assume the gas being ideal and use the ideal gas equation.

PV = nRT

Be careful of your units though. :)

You can even skip the right-hand side of the ideal gas equation, and just remember that the product of pressure and volume (i.e. the left side of the equation) remains constant as long as temperature doesn't change.

Thus, you can say PV(before)=PV(after), where the pressure and volume on the left side of the equation represent the conditions in the cylinder, while the right-hand side represents the conditions after the pressure has been decreased to standard pressure. Since you know both P and V on the left side, as well as P on the right side, that leaves V on the right side as the only unknown, and you simply need to solve for it.