Oxidation state of O in OH-

The oxidation state of oxygen (O) in the hydroxide ion (OH-) is -2.

In general, the oxygen atom in a hydroxide ion carries an oxidation state of -2. This is because the hydrogen atom in the ion has an oxidation state of +1, and the overall charge of the ion is -1. Therefore, in order to balance the charges, the oxygen atom must have an oxidation state of -2.