* O⁻ This represents the oxide anion, which has gained one electron. It's not the most common form.

* O²⁻ This is the oxide ion you'll most frequently encounter. It's a negatively charged oxygen atom that has gained two electrons.

* O₂ This is the formula for molecular oxygen, a neutral molecule made up of two oxygen atoms bonded together. It's not an ion.

* O₃ This is the formula for ozone, a molecule with three oxygen atoms. It's not an ion.

Key takeaway: The standard oxide ion you'll use in chemical formulas is O²⁻.