Oxygen molecule is

According to Molecular Orbital Theory (MOT),

σ1s²,σ∗1s²,σ2s²,σ∗2s²,(π2px²≈π2Py²),(π∗2Px¹≈π∗2Py¹)

The MOT diagram for O₂ molecule shows that there are two electrons each in the sigma 1s bonding, sigma 1s* antibonding, sigma 2s bonding, and sigma 2s* antibonding orbitals. The remaining valence electrons of each oxygen atom occupy two of the three degenerate pi orbitals, with one of the pi orbitals being completely filled with two electrons and the other two pi orbitals being half-filled, each with one electron.

The two unpaired electrons occupy two separate pi orbitals, which are higher in energy than the sigma orbitals. These pi orbitals have parallel spin, and their contribution to the overall magnetic moment of the molecule is in the same direction, making the O₂ molecule paramagnetic.