Oral metastases from osteosarcoma are rare, particularly in the soft tissues of the oral cavity. The aim of the current case is to present a patient with labial mucosa metastasis from a long bone osteosarcoma and review the literature. A 55-year-old man who had a recent leg amputation because of a giant cell tumor presented a lesion in the lower labial mucosa. After histopathologic and immunohistochemical analysis the diagnosis was of an undifferentiated sarcoma. The patient quickly developed other lesions on the scalp and on the hand, and biopsy of one of these lesions rendered the diagnosis of a giant cell-rich osteosarcoma. Reviewing all information, it was concluded that the leg tumor was the primary giant cell-rich osteosarcoma misdiagnosed as a giant cell tumor. This case emphasizes the importance of the general clinicians' multidisciplinary approach and association of information to arrive at the proper diagnosis, particularly in rare and difficult situations. Keywords: giant cells, labial mucosa, mouth, oral metastasis, osteosarcoma