Two electrochemical oscillations, previously called oscillation C and D, are observed for H2O2 reduction on a Pt electrode in an acidic solution when a small amount of halide ions is added to the solution. Detailed studies, including impedance analyses and in-situ light reflectance measurements as well as mathematical simulations, have revealed that both oscillations C and D fall into hidden negative differential resistance (HNDR) oscillators, though oscillation D can be classified into a new-type HNDR oscillator not reported thus far. The H2O2 reduction on Pt has two-type NDR's: One arises from a decrease in the coverage of adsorbed OH (acting as an autocatalyst for the H2O2 reduction) with a negative potential shift, and the other arises from suppression of the H2O2 reduction by formation of under-potential deposited hydrogen (upd-H) in further negative potentials. Oscillation C appears from hiding of the former NDR by a decrease in the coverage (Ox) of adsorbed halide ions (acting as a site blocking agent) with the negative potential shift. Oscillation D appears from hiding of the latter NDR by not only the decrease in Ox but also an addition of a transient cathodic current due to the upd-H formation, indicating that oscillation D is really a new-type HNDR oscillator.