Hydrothermal conversion studies were performed (bio-oil obtained under hydrothermal conditions) using Russian olive (Elaeagnus angustifolia L.) seeds as a waste biomass at different temperatures and different residence times. After the conditions where the highest yield of bio-oil was found, a mussel shell catalyst (which is a source of calcium oxide when calcined at T > 950 degrees C) was applied for the first time for the hydrothermal conversion of biomass. To compare and evaluate the catalytic performance of the catalyst, the effects of the catalyst on product distributions, elemental contents, and high heating values of products and bio-oil compositions were investigated. The use of the catalyst significantly increased the bio-oil and conversion yields. The bio-oils were analyzed by a gas chromatograph mass spectrometer (GC-MS). Characterization studies of the catalyst were completed by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, and X-ray diffraction (XRD) analysis. This study exclusively focuses on the bio-oil and solid residue yield to demonstrate the effects of the catalyst derived from a waste biomass. Suggestions for further studies are provided at the end of the Conclusion.