Osteomas of the maxilla and the mandible are rare, benign, and slow-growing tumors arising from the bone cells. They are more frequently located on the mandible than the maxilla, with mandibular body, angle, and condyle being more frequently affected. The development of a peripheral osteoma at the mandibular ramus is a rare occurrence, and few cases have been published in English literature. Even though the developmental process of this pathologic entity is characterized as slow growing, little data are known about the developmental rate of the peripheral osteomas of the mandible. In our paper, we present a case of peripheral osteoma of the external surface of the right mandibular ramus that was diagnosed 10 years earlier; its dimensions were measured precisely at that time with CT. The patient declined a surgical operation, and 10 years after the initial diagnosis, she visited our department with an increase in the size of the lesion and asked for a re-evaluation. A new CBCT was performed, and accurate measurement of the new dimensions of the lesion was conducted; this was compared with the previous dimensions. The comparison of the current CBCT with the CT performed 10 years earlier permitted to calculate the growth rate of osteoma that was approximately 2.0 mm/year.