Total number of electrons found in $O_{2}$ is $16 .$ But in $O_{2}^{2-}$ The total number of electrons is $18 .$ If we take the configuration of $O_{2}^{2-}$ using Molecular orbital Theory we get 

$\sigma_{2 \varepsilon}^{2} \sigma_{\star 2 \delta}^{2} \sigma_{2 z}^{2} \pi_{2 x}^{2} \pi_{2 y}^{2} \pi_{\star 2 x}^{2} \pi_{\star 2 y}^{2}$

Thus there are no unpaired electrons.