Based on Molecular Orbital Theory , how is it decided that the double bond in O2 molecule has one sigma bond and one pi bond?
$\ce{O2}$ MO diagram
From the MO diagram, we can see that it has 2 electrons in a $\sigma$ orbital, 4 in $\pi$ orbitals, and 2 in $\pi$* orbitals. Taking this together gives a bond order 2, with 1 coming from the sigma electrons and another 1 being the net value for the $\pi$ and $\pi$* electrons.