# According to MO theory, do 1s orbitals in dioxygen interfere?

According to MO theory, are the 1s orbitals in $$\ce{O2}$$ too radially contracted to interfere with each other and thus cannot form bonding and antibonding orbitals, or do they still do that?

It is always possible to define a bonding and antibonding orbital regardless of how small the overlap is. Because we solve the homonuclear diatomics in the symmetry of the molecule ($$D_\mathrm{\infty h}$$), the MOs we choose must respect that symmetry. It wouldn't matter if I put the oxygen atoms 10 light years apart, if I am treating them as part of the same quantum system, there are bonding and antibonding molecular orbitals. (Of course, at large distances bonding and antibonding orbital pairs becomes very near degenerate.)