Ab initio calculations at the MC/CASSCF level are used to determine in an accurate way the nature and position of the ground state of the KO molecule. The characteristic ²(Pi) and ²(Sigma)⁺ alkali monoxide ionic states show a theoretical energy separation of about 0.04 eV, which produces opposite results by authors in favour of one or the other symmetry. We test the basis set dependence and active space dependence of the calculated energetical ordering of both states, with an extensive study of the active orbitals selection; the results show a ground state of ²(Sigma)⁺ symmetry.