Introducion: Osteosarcoma is a primary malignant bone tumor most commonly found in long bones. However, in the oral cavity it mainly affects the ramus and angle of the mandible. Objetive: To present a case of osteolytic osteosarcoma of the mandible and discuss the difficulties of diagnosis and treatment. Case Report: A 22-year-old white female patient, non-smoker and non-drinker attended the stomatology service with complaints of increased volume on the right side of the mandible, exhibiting premolar and molar mobility in that area.Periapical and occlusal radiographs were performed and showed presence of radiolucent area and floating teeth, confirmed by a panoramic radiography showing involvement of the mandibular ramus. Two biopsies were performed in the region with inconclusive results for malignancy. The diagnostic hypothesis was osteosarcoma. A CT scan showed bone loss, floating teeth and involvement of areas adjacent to the tumor lesion and lymph nodes. The patient was referred to the oncology hospital and a frozen section biopsy was conclusive for osteosarcoma, with removal of the base and most of the right and left mandible ramus, followed by reconstruction with titanium plate. The tumor recurred and a new surgery, radiotherapy and chemotherapy were performed in an attempt to improve the clinical condition, with failure and death after four years of initial diagnosis and tumor mutation for rhabdomyosarcoma. Conclusion: Imaging and even histopathological outpatient exams were not able to promote the final diagnosis of osteosarcoma, impairing treatment and favoring recurrence and patient death. Key words: Osteosarcoma. Bone tumor. Diagnostic imaging.