There are lots of rules. Take a deep breath...
1. All atoms are treated as ions for this, even if they're covalently bonded.
2. Uncombined elements have an oxidation state of 0.
3. Elements just bonded to identical atoms, like O2 and H2, also have an oxidation state of 0.
4. The oxidation state of a simple monatomic ion, e.g. Na+, is the same as its charge.
5. In compounds or compound ions, the overall oxidation state is just the ion charge.
6. The sum of the oxidation states for a neutral compound is 0.
7. Combined oxygen is nearly always -2, except in peroxides where it's -1 (an in fluorides OF2, where its +2 and O2F2, where its +1 and in O2 where it's 0).
8. Combined hydrogen is +1, except in metal hydrides where it is -1 (and H2 where it's 0).
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