Oxyanions are anions with at least one oxygen atom bonded to another element, which we call the central atom. The central atom is usually nonmetallic and can for instance be chlorine, nitrogen, or sulfur. When we replace the negative charges of the oxyanion with hydrogen, we produce an oxyacid. When the acid dissociates in a solution, we get a weak, conjugate base. You can learn more about specific oxyanions and oxyacids in the following pages:
From the definitions above, we class hydroxide (OH-) as the simplest oxyanion. However, water is not an oxyacid as oxyacids must contain oxygen, hydrogen, and at least one other element. Thanks to IUPAC, we already have a standard name for the oxyanion hydroxide, but for more complex oxyanions we need specific IUPAC rules for oxyanion naming.