Oxidation State is an indicator of the degree of oxidation of an atom in a chemical compound

Rule 1:
A molecule containing only one element has an oxidation state of zero
E.g.
H2=0, O2=0

Rule 3:
For polyatomic species the sum of the oxidation states of all the atoms equals the charge on the ion or molecule
e.g.
NH3=0, NH4(+)=+1

Rule 5:
Fluorine is always -1

Oxidation leads to an increase in oxidation state, while reduction leads to a decrease in oxidation state