The oxidation number of a free element is always 0.
The oxidation number of a monatomic ion equals the charge of the ion.
Fluorine in compounds is always assigned an oxidation number of -1.
The alkali metals (group I) always have an oxidation number of +1.
The alkaline earth metals (group II) are always assigned an oxidation number of +2.
Oxygen almost always has an oxidation number of -2, except in peroxides (H2O2) where it is -1 and in compounds with fluorine (OF2) where it is +2.
Hydrogen has an oxidation number of +1 when combined with non-metals, but it has an oxidation number of -1 when combined with metals.
The algebraic sum of the oxidation numbers of elements in a compound is zero.
The algebraic sum of the oxidation states in an ion is equal to the charge on the ion.
Assigning oxidation numbers to organic compounds
The oxidation state of any chemically bonded carbon may be assigned by adding -1 for each more electropositive atom and +1 for each more electronegative atom, and 0 for each carbon atom bonded directly to the carbon of interest.