Assigning Oxidation Numbers

Start with the first rule and work down as necessary!

The sum of the oxidation numbers of each atom in a molecule must equal zero; the sum of the oxidation numbers of each atom in an ion must equal the charge on that ion.
Elements in their elemental form (atoms for metals, diatomic gases for H, N, O, etc.) have an oxidation number of 0.
Single atom ions have an oxidation number equal to their charge.
Alkali metals have an oxidation number of +1.
Alkaline earth metals have an oxidation number of +2.
Hydrogen's oxidation number is +1, unless it is combined with a metal in which case the oxidation number is -1.
Fluorine has an oxidation number of -1.
Oxygen usually has an oxidation number of -2. Exceptions are -1 in peroxides (O₂^2-), -1/2 in superoxides (O₂^-) and -1/3 in ozonides (O₃^-)