Bond order of 1.5 is shown by
Explanation
Configuration of O2
$$\sigma 1{s^2}{\sigma ^*}1{s^2}\sigma 2{s^2}{\sigma ^*}2{s^2}\sigma 2p_z^2\pi 2p_y^2{\pi ^*}2p_x^1{\pi ^*}2p_y^1$$
Bond order = $${{No.\,of\,{e^ - }\,in\,bonding\,M.O - No.\,of\,{e^ - }\,in\,antibonding\,M.O} \over 2}$$
Bond order of O2+ = $${{10 - 5} \over 2}$$ = 2.5
Bond order of O2- = $${{10 - 7} \over 2}$$ = 1.5
Bond order of O22- = $${{10 - 8} \over 2}$$ = 1
Bond order of O2 = $${{10 - 6} \over 2}$$ = 2