We investigated effects of AC electrical stress, the oil pressure and the insulation construction on the partial discharge (PD) characteristics of an oil-impregnated paper insulation system for oil-filled (OF) cables. The oil-impregnated paper insulation system with an oil gap was composed of oil-impregnated papers with a hole and papers without any hole. They were sandwiched between parallel plane electrodes. We observed PD signal under the conditions of the AC electrical stress from 8 to 13kV/mm and the oil pressure from 0.00 to 0.25MPaG, and the insulation thickness from about 1 to 2mm or the oil gap thickness from 0.125 to 0.625mm. As a result, it is firstly revealed that the AC electrical stress, the oil pressure and the oil gap thickness affect the continuity of PD occurrence. Then, the maximum PD charge magnitude and the PD repetition rate increased with the oil gap thickness. The lead time from PD ignition to the breakdown was not much influenced by the insulation thickness.