What is the volume of 80 g O_2 at STP?

At standard temperature and pressures, a molar quantity of an ideal gas has a volume of $22.4$ $L$.
So, all you have to do is calculate how many moles of oxygen you have (and oxygen like most gases save the Noble gases is diatomic), and multiply this by the molar volume at $273 K$, $22.4$ $L$ $m o {l}^{-} 1$.