Osteoid osteoma. Coloured X-ray of an osteoid osteoma (white, centre left) in the second phalange bone of the middle finger. Osteoid osteoma is a rare, benign tumour of bone tissue that causes deep pain. The tumour typically measures less than half a centimetre and most commonly affects the long bones of the arms and legs. Treatment includes aspirin to relieve pain and surgery to remove the abnormal bone.