What is the oxidation number of O in the ion OH-?
The oxidation number is a fictitious charge of atoms involved in chemical bonds that reflects electronegativity. The bond, the two electrons binding an element or ion are broken, with the charge, the electron assigned to the most electronegative atom. The sum of the oxidation numbers is equal to the charge on the ion or the molecule, and of course this charge might be ZERO...
Here we gots....
Here are some rules taken from a prior answer...they are included as reference. I do not propose that you learn them verbatim...
The ion has one oxygen atom and one hydrogen atom. Hydrogen is less electronegative than oxygen, and so will possess its usual
So, oxygen will have an oxidation number of