According to molecular orbital theory as bond order decreases stability of the molecule decreases.

Bond order = $1 \over 2$(N_b - N_a)

Bond order of $O_2^+$ = $1 \over 2$(10 - 5) = 2.5

Bond order of $O_2$ = $1 \over 2$(10 - 6) = 2

Bond order of $O_2^-$ = $1 \over 2$(10 - 7) = 1.5

Bond order of $O_2^{2-}$ = $1 \over 2$(10 - 8) = 1.0

Hence the correct order is $O_2^+$ > $O_2$ > $O_2^-$ > $O_2^{2-}$