If it contains a number in its formula (such as in peroxide, O2-) it is polyatomic as this means there are more than one atom of the same type. If it contains more than one element, which you would...
OH- is Polyatomic.
Hydroxide (OH-) contains both an atom of oxygen (O) and hydrogen (H). Many ions are from only ONE atom or element, but since OH- contains two separate elements, it is polyatomic.