Oxidation Rules

1. pure elements (stand alone elements) have an oxidation number of zero
2. for monoatomic ions (metal elements), the oxidation number is equal to its charge
3. fluorine alway has an oxidation number of -1 when in compounds
4. Cl, Br, and I always have an oxidation number of -1 in compounds, except when combined with oxygen or fluorine
5. the oxidation number of H is +1 and of O is -2 in most compounds.
• exceptions for H: in compounds with metals, H is -1
• exceptions for O: in peroxides, the charge is -1
6. the algebraic sum of the oxidation number for the atoms in a neutral compound must be zero; in a polyatomic ion, the sum must be equal to the charge of the ion

Here are a couple references that help explain:

http://www.dummies.com/how-to/content/rules-for-assigning-oxidation-numbers-to-elements.html

http://www.wikihow.com/Find-Oxidation-Numbers

element	usual oxidation state	exceptions
Group 1 metals	always +1
Group 2 metals	always +2
Oxygen	usually -2	except in peroxides and F2O (see below)
Hydrogen	usually +1	except in metal hydrides where it is -1 (see below)
Fluorine	always -1
Chlorine	usually -1	except in compounds with O or F (see below)