${\text{O}}^{2-}$ is isoelectronic with

Isoelectronic species are atoms, molecules, or ions that contain the exact same number of electrons.
Let us evaluate the total electron count for each option:

* The atomic number of Oxygen ($\text{O}$) is $8$. The oxide ion (${\text{O}}^{2-}$) has gained $2$ electrons:

$$\text{Electrons in }{\text{O}}^{2-}=8+2=10$$

* The atomic number of Fluorine ($\text{F}$) is $9$. The fluoride ion (${\text{F}}^{-}$) has gained $1$ electron:

$$\text{Electrons in }{\text{F}}^{-}=9+1=10$$

* The atomic number of Chlorine ($\text{Cl}$) is $17$: ${\text{Cl}}^{-}=17+1=18$ electrons.
* The atomic number of Lithium ($\text{Li}$) is $3$: ${\text{Li}}^{+}=3-1=2$ electrons.
* The atomic number of Potassium ($\text{K}$) is $19$: ${\text{K}}^{+}=19-1=18$ electrons.

Since both ${\text{O}}^{2-}$ and ${\text{F}}^{-}$ contain exactly $10$ electrons, they are isoelectronic with one another.