While filling electrons in π2px and π2pythe electronic configuration rules that one to be followed is

M.O.E order for O2 molecule [> 14e-]From the molecular orbitals energy order [MOE order] the π2px & π2py molecular orbitals [M.O's] have same energyi.e., π2px ≈ π2pythe filling of electrons  in these two molecular orbitals is done in according to Hund's Rule.→ According to Hund's Rule, all the degenerate orbitals are occupied by one electron then  only pairing of electrons  takes place in degenerate orbitals.