Abstract: To address the quality deterioration of Japanese flounder (Paralichthys olivaceus) muscle characterized by the formation of a paste-like texture during frozen storage at −20 °C, we investigated the antifreeze mechanism and optimal concentration of oregano (Origanum vulgare) essential oil. Healthy P. olivaceus with a body mass of (700±50) g were used as experimental subjects, and four groups were established: a control group (Soaked in normal saline) and three oregano essential oil soaking groups at concentrations of 100, 500, and 1000 mg·kg⁻¹ (Mass fractions of the key components: carvacrol ≥5.0%, thymol ≥5.0%, eugenol ≥2.5%, cinnamaldehyde ≥4.0%). The samples were stored at −20 °C for 28 d, and indicators including texture properties, moisture-related parameters, myofibrillar protein oxidation and denaturation, antioxidant capacity, and microstructure were measured every 7 d. The results show that after 28 days of frozen storage, the control group exhibited significant quality deterioration (p<0.05): hardness, springiness, and chewiness decreased by 46.21%, 27.73%, and 38.86%, respectively; the drip loss reached (17.49±1.01)%; water-holding capacity (WHC) decreased to 84.36 ± 0.85%; the carbonyl mass fraction of myofibrillar protein increased by 82.74%; the activities of superoxide dismutase (SOD) and catalase (CAT) decreased by 62.90% and 65.56%, respectively; malondialdehyde (MDA) concentration increased to 3.98 ± 0.19 nmol·mg⁻¹; the muscle fiber structure was disordered with severe degradation of the endomysium; and collagen mass fraction decreased to (0.251±0.021) mg·g⁻¹ (A 58.96% reduction compared with the initial value). The 100 and 500 mg·kg⁻¹ oregano essential oil groups significantly ameliorated the above deterioration (p<0.05), among which the 500 mg·kg⁻¹ group showed the best effect: on 28^th day, its hardness was 24.97% higher than that of the control group, Ca²⁺-ATPase activity increased by 26.69%, and muscle fibers still maintained a partial hexagonal arrangement. The 100 mg·kg⁻¹ group had a 17.56% lower drip loss than that of the control group, and its collagen mass fraction was 26.2% higher than that of the control group (p<0.05). In contrast, the 1 000 mg·kg⁻¹ group exhibited inferior protective effects compared with the medium-low concentration groups. In conclusion, medium-low concentrations (100, 500 mg·kg⁻¹) of oregano essential oil can alleviate the quality deterioration of frozen P. olivaceus muscle, possibly by regulating ice crystal morphology, inhibiting oxidation, and reducing myofibrillar protein degradation.