In this work, the hydrothermal conversion of oil shale from Natih B Formation (Oman) was evaluated at different temperatures (300, 350 and 400 °C) and 2291.6 psi (close to the reservoir pressure) using both crushed and uncrushed core samples. The performance of hydrothermal conversion of oil shale was evaluated in terms of material balance (yield of synthetic oil and gas), composition of evolved gases, synthetic oil quality (SARA fractions, elemental analysis, alkanes distribtuion by GC, and aliphtics and aromatic structure change by GC–MS etc.), and the change in organic matter (OM) in the core by LF-NMR. In addition, the change in pore size and structure was also investigated by X-ray computed tomography. The results show that 350 °C gives the best performance for the hydrothermal conversion of oil shale to generate synthetic oil in terms of synthetic oil yield and quality. The high quality of synthetic oil is manifested by a much higher content of aliphatic hydrocarbons than aromatics; a higher amount of saturates and aromatics (light fractions) than resins and asphaltenes (heavy fractions); and a higher amount of light alkanes (especially C₁₀-C₂₀) with a lower amount of heavy alkanes (especial C₂₆-C₃₆). The hydrothermal conversion of OM was confirmed by LF-NMR technique. It was found that hydrothermal treatment induced a significant increase in pore size and porosity with the formation of connected pore channels, resulting in the transformation of oil shale from an extremely low permeable into a higher permeable porous medium. This change in rock properties benefits the subsequent injection of any fluids for the conversion and production of oil shale in field. These promising results technically prove the potential for the development of shale oil from Natih B Formation by hydrothermal conversion.