Young’s modulus experiments have been carried out to study the kinetics of ab-plane crack propagation in single domain YBa2Cu3Ox (YBCO) during a prolonged oxygen heat treatment at 400 °C up to 188 h. It has been found that the modulus value experiences a rapid fall between the annealing time 48 and 96 h. X-ray diffraction (XRD) experiments have been carried out to investigate the structural phase transition during the oxygenation process. Consistently, a diffraction peak shift has been observed in this region that indicates a massive tetragonal-to-orthorhombic (T-to-O) transition. SQUID magnetization measurements of annealed samples have shown clear oxygen inhomogeneity in this regime suggesting a T-to-O phase boundary in the crystal. A physical model is provided to describe the operating mechanism of crack propagation in single domain YBCO.