Weakly ionized plasma of a pulsed-discharge afterglow in oxygen at low pressures (0.05-0.15 torr) is investigated using probe diagnostics. The plasma conductivity is measured by supplying an additional probing current pulse at a certain instant during the afterglow. The spectral line intensities are also measured to additionally monitor the densities of charged particles. The measurements of the time behavior of the electron density in an oxygen afterglow plasma confirm the previous conclusion that the electrons escape due to enhanced diffusion, which results in the formation of an ion-ion plasma. The possibility of realizing the opposite ultimate case - the detachment decay regime with an increase in the electron density to the density of positive ions in the first stage and the transition to the electron-ion plasma in the second stage - is demonstrated.