Four electrochemical oscillations of different types (named oscillations (alpha, beta, gamma, and delta), which are different from previously reported Frumkin-type oscillations in low ionic strength electrolytes, appear for peroxodisulfate (S2O82-) reduction on Pt and Au electrodes under high ionic strength conditions. Impedance measurements for oscillation gamma that appears in a region of the most positive potentials have shown that it is classified as hidden negative differential resistance (hidden-NDR or HNDR) oscillators. Detailed electrochemical measurements have suggested that the NDR arises from a decrease (with a negative potential shift) in the surface coverage of adsorbed OH, which acts as a catalyst for the dissociative adsorption of S(2)O(8)2- (the first step of its reduction). The NDR is hidden by the desorption (with the negative potential shift) of adsorbed sulfate (SO42-) produced by the dissociative adsorption and reduction of S2O82-, which works as a siteblocking agent. Mathematical simulation based on this model has reproduced the appearance of oscillation gamma. The result supports our previously proposed concept of autocatalytic effect of adsorbed OH and catalytic effect of adsorbed halogen on the dissociative adsorption of H2O2, showing the generality of the electrocatalytic effect of adsorbed OH and halogen.