EHV/UHV oil-impregnated paper bushings are important insulating and current-carrying components of key equipment such as large power transformers and converter transformers, and they are exposed to multi-physics field conditions characterized by strong electric fields, high temperatures, and strong mechanical stresses for long periods. Once internal defects evolve into high-energy arc faults, explosion and combustion may be triggered, and power grid security as well as personnel safety may be seriously threatened. The aging mechanisms of the internal insulation of oil-impregnated paper bushings were systematically reviewed, and the variation laws of different characteristic quantities used to characterize insulation defects were summarized. Around characteristic parameters including partial discharge, dielectric response, dissolved gases in oil, temperature, and pressure, the development status and applicability boundaries of electrical-parameter, non-electrical-parameter, and multi-parameter monitoring technologies were sorted out. The observable features and detection methods in the spark discharge stage and the early arc stage were summarized, and pressure signals were considered to have the strongest correlation with high-energy arc faults. Based on the review analysis, subsequent research should focus on establishing a unified aging mechanism model of “multi-field coupling under real operating conditions-microscopic reaction pathways-macroscopic measurable parameters”, improving multi-parameter diagnostic criteria and graded early-warning thresholds under complex and concurrent fault conditions, carrying out full-scale arc-fault tests of EHV/UHV bushings in coordination with multi-scale simulations, refining the full-process mechanism of “arc discharge-gas generation in insulating oil-pressure wave propagation”, thereby providing guidance for the establishment of a multi-parameter integrated monitoring system and an active defense framework for EHV/UHV oil-impregnated paper bushings.