Over recent years, oligomer ionic liquids (OILs), a novel class of ionic liquids, are becoming preferential electrolytes for high-performance energy-storage devices, such as supercapacitors with enhanced energy density and non-flammable lithium-ion batteries (LIBs). Herein, structures, properties, and their associations of the up-to-the-minute formulated OILs are systematically summarized and elaborately interpreted, especially for dicationic ionic liquids and tricationic ionic liquids. The physicochemical and electrochemical properties of OIL-based electrolytes are presented and analyzed, which are vitally important for supercapacitors and LIBs. Subsequently, the applications of OILs as electrolytes for supercapacitors and LIBs are summarized, with the comparisons of the energy-storage mechanisms and performance between OILs and MILs electrolytes in supercapacitors. Meanwhile, the optimization of the dynamic performance of OILs electrolytes is provided. Finally, the main difficulties and probable perspectives of OIL-based electrolytes are presented for future work. This review would contribute to a deep understanding of OILs and design optimized OIL-based electrolytes for energy storage systems.