Osteoid osteoma. X-ray of an osteoid osteoma (lower centre) in the humerus bone of a patient's upper arm. The shoulder appears at the top and the elbow at the bottom. Osteoid osteoma is a rare, benign tumour of bone tissue that causes deep pain. The tumour typically measures less than half a centimeter and most commonly affects the long bone of the leg or arm, as here. Treatment includes aspirin to relieve pain and surgery to remove the abnormal bone.