A species is paramagnetic if it contains one or more unpaired electrons. According to MOT, the configuration of $O_2$ (16 electrons) is: $\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\sigma 2p_z^2(\pi 2p_x^2=\pi 2p_y^2)({\pi }^{\ast }2p_x^1={\pi }^{\ast }2p_y^1)$

• There are two unpaired electrons in the subshell ${\pi }^{\ast }2p$.
• In contrast, $N_2,F_2$, and $C_2$ (at least in their ground state configurations) have all electrons paired, making them diamagnetic.