Hello,
Diatomic, of course, means that each oxygen molecule contains 2 oxygen atoms (ie, O2). Cool. So you know you have 56 liters of a gas, oxygen here, at STP. Do you have a formula that would allow you to calculate how many molecules of O2 gas would occupy that much volume? After you calculate that, its a simple matter of finding out how many electrons each O2 molecule contains, and multiplying your number of molecules by the number of electrons in each molecule by the total charge of each electron.
Hope this helps, reply if you need more clarification! I've deliberately avoided simply giving you the formulas, of course.