<p>Osteoma is a benign slow growing bone tumour with predominance in long bones, and is rare in the skull. They are a rare occurrence in the mastoid temporal bone, being usually asymptomatic and treated mainly for cosmetic reasons. We report a case of a young female who presented with a history of swelling in the left post aural region since 10 years. Swelling was gradually increasing in size and was painless. Patient underwent excision of the tumour without any significant complication. As per our knowledge this appears to be the largest tumour reported in literature. The clinical presentation and radiological features of osteoma are characteristic but differential diagnosis should include eosinophilic granuloma, giant cell tumour, monostotic fibrous dysplasia, solitary variant of multiple osteoma, and osteoblastic metastasis. Osteomas present on the mastoid or squamous portion of the temporal bone need to be dealt for cosmetic purposes or if they are causing symptoms, while surgery should include careful removal of periosteal cover and safe margin of the mastoid cortex around it. </p>