According to M.O theory the number of molecular orbitals in O2 are:

Explanation

Oxygen atoms contribute ground-state molecular orbitals from 1s, 2s, and 2p electrons. The 1s orbitals form σ1s and σ*1s filled Mos. The 2s orbitals form σ2s and σ*2s filled MOs. The σ2p and π2p MOs are also filled, while the antibonding π*2p orbitals are partially filled in triplet oxygen (the ground state), or only one is filled in singlet oxygen. But the antibonding σ*the 2p orbital is ordinarily empty in the ground state, and higher orbitals are not filled unless the oxygen molecule absorbs light energy which is capable of exciting the electrons to higher orbitals.