When combined with metals such as, for example, sodium or calcium, the atom oxygen is given to oxidation state of negative two. But there's an exception. It's given oxidation state of negative one when combined in the form H 202. But that's because this rule is more important than this rule. So on this table, every rule that comes before it is more important. For example, this is the most important rule, the second most important rule, third most important rule, and fourth most important rule.