The co-pyrolysis characteristics of oil shale-biomass blends were investigated by open-system pyrolysis with a thermogravimetry-mass spectrometry (TG-MS) analyzer and by closed-system pyrolysis with a fixed-bed reactor. Online Fourier transform infrared (FTIR) spectrometry and gas chromatography-mass spectrometry (GC-MS) were employed to analyze the gas and liquid products that were generated from the closed-system pyrolysis. In addition, the Gaussian program was also employed for quantum chemistry calculations. According to the results, one important reaction mode of oil shale-biomass interactions can be described as the "biogas-kerogen" reaction, which means the combination of kerogen and some free radicals volatilized from the biomass in the temperature range of 180-400 degrees C. During the co-pyrolysis, the oil shale influences the distribution of the oxygen content in the biomass products, causing an increase of CO2 and CO in the gaseous products and a reduction of oxygen-containing compounds such as alcohols, aldehydes, lipids, and ethers in the liquid products. The results will be useful for improving the quality of shale oil in the industrial utilization of oil shale in the future.