The influence of the number of accounted admixed configurations on the accuracy of the energy spectra of the ground and four excited configurations (1s22s2p5 and 1s22s22p33l) as well as on the radiative lifetimes of the excited levels is investigated. The five ions (Mg V, Si VII, Cl X, Ca XIII and Cr XVII) of the oxygen isoelectronic sequence are taken as examples. The calculations are performed on the basis of the transformed radial orbitals. The admixed configurations are selected according to their influence evaluated in the second order of perturbation theory.