The etiology of os odontoideum is multifactorial considering the associated abnormalities, reports of congenital-familiar occurrence, and early childhood craniovertebral trauma which also plays a role in the etiology. Patients with reducible lesions require stabilization. Asymptomatic patients are at risk for later instability. Patients who underwent childhood C1-C2 fusion must be followed for later problems. The irreducibility was seen due to trapped transverse ligament, pannus, or previous dorsal C1-C2 fusion.