Wood is an attractive renewable material, yet its mechanical performance, dimensional stability, and moisture sensitivity limit its use in structural applications. Conventional densification methods often rely on aggressive alkali solutions or resin impregnation, generating environmental concerns and causing excessive cell-wall degradation. In this study a sustainable hydrothermal densification process for oak wood using only a water–ethanol solution in a sealed Teflon-lined autoclave has been investigated and implemented. Treatment at 195 °C for 80 min selectively degraded hemicelluloses and partially reorganized lignin without compromising the cellulose network, followed by hot-pressing at 100 °C under 5 MPa. The hydrothermally densified oak exhibited a significant improvement in stiffness, with a 125% increase in storage modulus relative to untreated wood and a 19.6% enhancement compared to alkali-treated samples. The findings demonstrated that hydrothermal processing is an efficient, low-impact alternative for producing high-performance densified wood suitable for sustainable construction and composite applications.