We present a critical analysis of data available in literature on the degradation of low-pressure oxygen plasma. Special attention is paid to two effects repeatedly observed in the initial stage of plasma degradation: a sharp growth in the flux of negative ions incident on the wall and an increase in the probing current during the laser-induced photodetachment. Both these phenomena are attributed by various authors to an absolute growth in the negative ion concentration. Our analysis shows that the whole body of experimental data cannot be explained based only on the influence of bulk plasmachemical reactions without taking into account special features of the transport processes in electronegative gases. An alternative mechanism is proposed which involves a two-stage degradation of the electronegative gas plasma accompanied by a change in the diffusion regime.