Rules to follow: 1:. Oxidation number of elements is 0. 2:. Oxidation number of oxygen in compounds is always -2. 3:. Hydrogen in compounds is +1, except for hydrides (-1). 4:. Numbers in on the left of elements/compounds can be ignored- these are only used to balance the chemical equation and do not affect oxidation number. 5:. Oxidation number is the most consistent way to determine if a reaction is redox. 6:. For good schools, some teachers may choose to set questions where the oxidation number of an atom is 0.5 or 1.5. Don't doubt yourself if you follow rules 1 to 5.