The polyatomic ion containing the greatest number of oxygen atoms is the peroxodisulfate ion (S₂O₈²⁻). It has a total of 8 oxygen atoms.

Let's break down why:

* Peroxodisulfate ion (S₂O₈²⁻): This ion has two sulfur atoms and eight oxygen atoms.

* Other common polyatomic ions with multiple oxygen atoms:

* Sulfate ion (SO₄²⁻): 4 oxygen atoms

* Phosphate ion (PO₄³⁻): 4 oxygen atoms

* Nitrate ion (NO₃⁻): 3 oxygen atoms

* Carbonate ion (CO₃²⁻): 3 oxygen atoms

While there are other polyatomic ions with even more oxygen atoms, they are less commonly encountered in basic chemistry.