Using molecular orbital theory, the bond order of the $\mathrm{O_2}$ molecule is:

$\mathrm{O_2}$ has 16 electrons. The MO configuration (filling bonding before antibonding within each shell) is: $\sigma_{1s}^2 \sigma_{1s}^{*2} \sigma_{2s}^2 \sigma_{2s}^{*2} \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \pi_{2p_x}^{*1} \pi_{2p_y}^{*1}$ Bonding electrons: $2 + 2 + 2 + 2 + 2 = 10$. Antibonding: $2 + 2 + 1 + 1 = 6$. Bond order $= \dfrac{N_b - N_a}{2} = \dfrac{10 - 6}{2} = 2$. The two unpaired electrons in the $\pi^*$ orbitals also explain why $\mathrm{O_2}$ is paramagnetic, a fact Lewis theory completely fails to predict.