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The low-gain free-electron laser (FEL) oscillators are cutting-edge tools to produce fully coherent radiation in the spectral region from terahertz to vacuum ultraviolet, and potentially in hard x ray. In this paper, it is proposed to utilize an FEL oscillator with multistage undulators to enable gain cascading in a single pass, making it possible to achieve shorter single pulse lengths, higher peak power, and even higher pulse energy than the normal FEL oscillator. Theoretical analysis and numerical simulations in the infrared and hard x-ray regions show that our proposal is effective.