This article highlights that the long-term and serial follow-up of adolescents with osteoid osteoma should be considered, even after complete excision of the nidus owing to the possibility of the delayed onset or progression of femur head and neck deformities or osteoarthritis. It is important to recognize the sequelae of osteoid osteomas, such as bone edema and new bone formation, which can alter the normal anatomy of the proximal femur. We report two cases of osteoid osteoma in the proximal femur, which showed progressive hypertrophy of the femoral neck after excision of the nidus and subsequent cam-type femoroacetabular impingement (FAI), requiring additional osteochondroplasty procedures. Even though hip pain was relieved immediately after excision of the nidus in both cases, cam-type FAI developed during postoperative follow-up of 18 months (case 1) and 6.5 years (case 2). Hip pain subsided within 1 month of osteochondroplasty, and the full range of motion of the hip joint was achieved and was being maintained after postoperative follow-up of 1 year (case 1) and 6 months (case 2).