Each atom of a pure element has an oxidation number of zero(0).
For monatomic ions, the oxidation number equals the charge on the ion.
Fluorine always has an oxidation state of -1 in compounds.
Cl, Br, and I always have oxidation numbers of -1, except when combined with oxygen or fluorine.
The oxidation number of H is +1 and O is -2 in most compounds.
The sum of the oxidation numbers must equal the charge on the molecule or ion.