Several studies have explored the potential of optoacoustic imaging for monitoring thermal therapies, yet the origin of the contrast in the images is not well understood. A technique is required to measure the changes in the optical and thermomechanical properties of tissues upon coagulation to better understand this contrast. An interferometric method is presented for measuring simultaneously the optical and thermomechanical properties of native and coagulated ex-vivo bovine tissue samples based on analysis of the surface displacement of irradiated samples. Surface displacements are measured after irradiation by short laser pulses at 750 nm. A 51% decrease in the optical attenuation depth is observed for coagulated liver samples compared to native samples. No significant differences in the Grüneisen coefficient are measured in the native and coagulated tissue samples. A mean value of 0.12 for the Grüneisen coefficient is measured for both native and coagulated liver tissues. The displacement profiles exhibit consistent differences between the two tissue types. To assess the changes in the sample mechanical properties, the experimental data also are compared to numerical solutions of the equation for thermoelastic deformation. The results demonstrate that differences in the tissue expansion dynamics arise from higher values of elastic modulus for coagulated liver samples compared to native ones.