Fe (Iron) is a transition metal and their charge varies from metal to metal, you just got to memorize it unless you're taking advanced chemistry and learn why.
Fe (iron) will either get the charge Fe2+ or Fe3+. Unfortunately I can't subscript the numbers here, that would have made it a lot easier.
I'll explain some more here before I go on to oxygen.
All atoms wants to have 8 electrons in their outer shell, this is a stable electron distribution. Now I'll go over to the groups:

Groups:
1 – Alkaline metals will always get the charge 1+. This is because it has 1 electron in it's outer shell and it's easier to give one away than take 7.
O (Oxygen) is in group nr. 16, which means it will _always_ get the charge O2-.
2 – Earth alkaline metals will always get the charge 2+ of the same reasons as group 1. Easier to give away one, than to take 7 to you.
Now here's the tricky ones:
Groups 2 – 14 doesn't follow this rule and only partly 15, 16 and 17.

Groups 2 – 14 consists of the transition metals, these you just got to memorize as said earlier. They can get several charges and if you want to decide which one the transition metal you're looking for has got, you need to look at the other atom(s) in the molecule. I'll explain this later.
Groups 15 – 18 got a rule, but only applies to some of the elements.
Group 15: Nitrogen gets the charge 3-. It has 5 electrons in its outer shell and it's easier to take 3 than to give away 5.
Group 16: Oxygen and sulfur will get the charge 2- because of the same reason as explained.
Group 17: Fluoride, chlorine, bromine and iodine will get the charge 1-.
I know it's hard to remember which in each group which has that charge and so on, but in group 15-17 you start on the top. and go down.
15 got 1
16 got 2
17 got 4
Group 18 consists of the noble gases and they have 8 electrons in their outer shell, so they don't change charge at all!
And hydrogen gets the charge 1+.

Now let's say you get FeO
Hmmm, Fe can either get the charge 2+ or 3+ and Oxygen gets the charge 2-. Now you need to look which one fits.
2 – 2 = 0, so it's stable!
One more:
CoCl2 (One cobalt atom and two chlorine):
Co is a transition metal and can have the charges 2+ and 3+, chlorine is in group 17 and therefor has 1-. Since it's two chlorine atoms it's 2- and then it has to be Co2+.

It's more difficult to go the other way around, but I'll try to explain it to you:
Build a salt out of sodium and oxygen:
Well, Na is in group 1, so it will have the charge +1.
Oxygen is in the 16th group and will therefor have the charge -2. Now you got to find the common denominator.
1 – 2 = -1, so that doesn't work.
2 – 2 = 0, it works!
So it's Na2O (two sodium and one oxygen).
So you got to use two natrium molecules, simple as that. Always find the common denominator and you can figure it out.

Hope it helps and good luck passing chemistry!

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