Answer
$O^{2-}$ is larger than $O$. Anions are larger than their corresponding neutral atoms, since the addition of electrons screens the attraction from the nucleus, giving the electrons more chance to be further from the nucleus, as a result increasing the radium of the atom.
Work Step by Step
To change a neutral atom into an anion, electrons must be added to the neutral atom.
Adding more electrons means there are more electron-electron repulsions among the electrons, screening the attraction from the nucleus.
At the same time, adding electrons does not change the nuclear charge, so the attraction from the nucleus does not change.
Therefore, adding more electrons to a neutral atom gives the electrons more freedom to move around, meaning there is more chance for an electron to be further from the nucleus. So, the radium would increase when an atom turns into an anion.
In conclusion, anions are larger than their corresponding neutral atoms. That is why $O^{2-}$ is larger than $O$.
